Update systemctl en/disable to start/stop service (#12100)

If there is a combination of `systemctl` commands where in one you are
enabling it and in another you are starting it, that can be done in one
single command with an additional flag called `--now`. This commit
changes those two command steps in docs to one command.

The help of systemctl shows the following explanation of `--now`:

```
Start or stop unit in addition to enabling or disabling it
```

Signed-off-by: Suraj Deshmukh <surajd.service@gmail.com>

content/en/docs/setup/independent/install-kubeadm.md

@@ -156,7 +156,7 @@ sed -i 's/^SELINUX=enforcing$/SELINUX=permissive/' /etc/selinux/config
 
 yum install -y kubelet kubeadm kubectl --disableexcludes=kubernetes
 
-systemctl enable kubelet && systemctl start kubelet
+systemctl enable --now kubelet
 ```
 
   **Note:**
@@ -210,7 +210,7 @@ curl -sSL "https://raw.githubusercontent.com/kubernetes/kubernetes/${RELEASE}/bu
 Enable and start `kubelet`:
 
 ```bash
-systemctl enable kubelet && systemctl start kubelet
+systemctl enable --now kubelet
 ```
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