Fix errors in configure-multiple-schedulers.md (#8540)

* Fix errors in configure-multiple-schedulers.md

- Failed to push docker image
- Errors found in my-scheduler logs
 
```
...
E0515 06:36:28.180428       1 reflector.go:205] k8s.io/kubernetes/vendor/k8s.io/client-go/informers/factory.go:130: Failed to list *v1beta1.ReplicaSet: replicasets.extensions is forbidden: User "system:serviceaccount:kube-system:default" cannot list replicasets.extensions at the cluster scope
```

* Replace double quote with backticks for code strings

* Use `system:kube-scheduler` instead of `cluster-admin`

* Create dedicated service account for `my-scheduler`

content/en/docs/tasks/administer-cluster/my-scheduler.yaml

@@ -1,3 +1,22 @@
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: my-scheduler
+  namespace: kube-system
+---
+kind: ClusterRoleBinding
+apiVersion: rbac.authorization.k8s.io/v1
+metadata:
+  name: my-scheduler-as-kube-scheduler
+subjects:
+- kind: ServiceAccount
+  name: my-scheduler
+  namespace: kube-system
+roleRef:
+  kind: ClusterRole
+  name: kube-scheduler
+  apiGroup: rbac.authorization.k8s.io
+---
 apiVersion: apps/v1
 kind: Deployment
 metadata:
@@ -19,6 +38,7 @@ spec:
         tier: control-plane
         version: second
     spec:
+      serviceAccountName: my-scheduler
       containers:
       - command:
         - /usr/local/bin/kube-scheduler