Tweak kubeadm-upgrade page for indentation nits

The indentation on the page is "messy". We don't encourage implicit code blocks (4-space indentations). The nesting of contents are not properly expressed either.

content/en/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade.md

@@ -83,14 +83,18 @@ Pick a control plane node that you wish to upgrade first. It must have the `/etc
 
   {{< tabs name="k8s_install_kubeadm_first_cp" >}}
   {{% tab name="Ubuntu, Debian or HypriotOS" %}}
+  ```shell
    # replace x in {{< skew currentVersion >}}.x-00 with the latest patch version
    apt-mark unhold kubeadm && \
    apt-get update && apt-get install -y kubeadm={{< skew currentVersion >}}.x-00 && \
    apt-mark hold kubeadm
+   ```
   {{% /tab %}}
   {{% tab name="CentOS, RHEL or Fedora" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-0 with the latest patch version
   yum install -y kubeadm-{{< skew currentVersion >}}.x-0 --disableexcludes=kubernetes
+  ```
   {{% /tab %}}
   {{< /tabs >}}
   <br />
@@ -149,13 +153,13 @@ Failing to do so will cause `kubeadm upgrade apply` to exit with an error and no
 
 Same as the first control plane node but use:
 
-```
+```shell
 sudo kubeadm upgrade node
 ```
 
 instead of:
 
-```
+```shell
 sudo kubeadm upgrade apply
 ```
 
@@ -176,14 +180,18 @@ Also calling `kubeadm upgrade plan` and upgrading the CNI provider plugin is no
 
   {{< tabs name="k8s_install_kubelet" >}}
   {{% tab name="Ubuntu, Debian or HypriotOS" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-00 with the latest patch version
   apt-mark unhold kubelet kubectl && \
   apt-get update && apt-get install -y kubelet={{< skew currentVersion >}}.x-00 kubectl={{< skew currentVersion >}}.x-00 && \
   apt-mark hold kubelet kubectl
+  ```
   {{% /tab %}}
   {{% tab name="CentOS, RHEL or Fedora" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-0 with the latest patch version
   yum install -y kubelet-{{< skew currentVersion >}}.x-0 kubectl-{{< skew currentVersion >}}.x-0 --disableexcludes=kubernetes
+  ```
   {{% /tab %}}
   {{< /tabs >}}
   <br />
@@ -215,14 +223,18 @@ without compromising the minimum required capacity for running your workloads.
 
   {{< tabs name="k8s_install_kubeadm_worker_nodes" >}}
   {{% tab name="Ubuntu, Debian or HypriotOS" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-00 with the latest patch version
   apt-mark unhold kubeadm && \
   apt-get update && apt-get install -y kubeadm={{< skew currentVersion >}}.x-00 && \
   apt-mark hold kubeadm
+  ```
   {{% /tab %}}
   {{% tab name="CentOS, RHEL or Fedora" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-0 with the latest patch version
   yum install -y kubeadm-{{< skew currentVersion >}}.x-0 --disableexcludes=kubernetes
+  ```
   {{% /tab %}}
   {{< /tabs >}}
 
@@ -249,14 +261,17 @@ without compromising the minimum required capacity for running your workloads.
 
   {{< tabs name="k8s_kubelet_and_kubectl" >}}
   {{% tab name="Ubuntu, Debian or HypriotOS" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-00 with the latest patch version
   apt-mark unhold kubelet kubectl && \
   apt-get update && apt-get install -y kubelet={{< skew currentVersion >}}.x-00 kubectl={{< skew currentVersion >}}.x-00 && \
   apt-mark hold kubelet kubectl
   {{% /tab %}}
   {{% tab name="CentOS, RHEL or Fedora" %}}
+  ```shell
   # replace x in {{< skew currentVersion >}}.x-0 with the latest patch version
   yum install -y kubelet-{{< skew currentVersion >}}.x-0 kubectl-{{< skew currentVersion >}}.x-0 --disableexcludes=kubernetes
+  ```
   {{% /tab %}}
   {{< /tabs >}}
   <br />
@@ -279,8 +294,8 @@ without compromising the minimum required capacity for running your workloads.
 
 ## Verify the status of the cluster
 
-After the kubelet is upgraded on all nodes verify that all nodes are available again by running the following command
+After the kubelet is upgraded on all nodes verify that all nodes are available again by running
-from anywhere kubectl can access the cluster:
+the following command from anywhere kubectl can access the cluster:
 
 ```shell
 kubectl get nodes
@@ -296,6 +311,7 @@ This command is idempotent and eventually makes sure that the actual state is th
 To recover from a bad state, you can also run `kubeadm upgrade apply --force` without changing the version that your cluster is running.
 
 During upgrade kubeadm writes the following backup folders under `/etc/kubernetes/tmp`:
+
 - `kubeadm-backup-etcd-<date>-<time>`
 - `kubeadm-backup-manifests-<date>-<time>`
 
@@ -334,3 +350,4 @@ and post-upgrade manifest file for a certain component, a backup file for it wil
 
 - Fetches the kubeadm `ClusterConfiguration` from the cluster.
 - Upgrades the kubelet configuration for this node.
+