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/configure-multiple-schedulers.md

@@ -44,7 +44,7 @@ For more details, please read the GCR
 [documentation](https://cloud.google.com/container-registry/docs/).
 
 ```shell
-docker build -t my-kube-scheduler:1.0 .
+docker build -t gcr.io/my-gcp-project/my-kube-scheduler:1.0 .
 gcloud docker -- push gcr.io/my-gcp-project/my-kube-scheduler:1.0
 ```
 
@@ -63,6 +63,9 @@ config. Save it as `my-scheduler.yaml`:
 An important thing to note here is that the name of the scheduler specified as an
 argument to the scheduler command in the container spec should be unique. This is the name that is matched against the value of the optional `spec.schedulerName` on pods, to determine whether this scheduler is responsible for scheduling a particular pod.
 
+Note also that we created a dedicated service account `my-scheduler` and bind the cluster role
+`system:kube-scheduler` to it so that it can acquire the same privileges as `kube-scheduler`.
+
 Please see the
 [kube-scheduler documentation](/docs/admin/kube-scheduler/) for
 detailed description of other command line arguments.