Fix a wrong explaining text about running command to create replication controllers.

At [Step Three Turn up the replicated slave pods.] (https://github.com/GoogleCloudPlatform/kubernetes/blob/master/examples/guestbook/README.md#step-three-turn-up-the-replicated-slave-pods), 

It is described like below, 
```
to create the service by running:
$ cluster/kubecfg.sh -c examples/guestbook/redis-slave-controller.json create replicationControllers
```

But in this place,  create a replication controller, I guess should be like below.
```
to create the replication controller by running:
```