Add '--namespace' to rolebinding creation

Our current rolebinding docs on https://docs.couchbase.com/operator/1.2/install-kubernetes.html show the command as follows:

kubectl create rolebinding couchbase-operator --role couchbase-operator --serviceaccount default:couchbase-operator

This will create the rolebinding ALWAYS in the current namespace.

For users it's not obvious that as well as substituting the 'default' in the serviceaccount reference, you need to also make sure it's in the same namespace too.

To make this simpler, we should just add it.