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`

test/examples_test.go

@@ -369,7 +369,7 @@ func TestExampleObjectSchemas(t *testing.T) {
 			"memory-defaults-pod":                        {&api.Pod{}},
 			"memory-defaults-pod-2":                      {&api.Pod{}},
 			"memory-defaults-pod-3":                      {&api.Pod{}},
-			"my-scheduler":                               {&extensions.Deployment{}},
+			"my-scheduler":                               {&api.ServiceAccount{}, &rbac.ClusterRoleBinding{}, &extensions.Deployment{}},
 			"namespace-dev":                              {&api.Namespace{}},
 			"namespace-prod":                             {&api.Namespace{}},
 			"persistent-volume-label-initializer-config": {&admissionregistration.InitializerConfiguration{}},