Add kuberay operator addon to cmd in gke-gcs-bucket.md

[parnurzeal]

Without this option, the kubectl apply cmd won't work as it doesn't know the CRD.

Why are these changes needed?

The steps in the doc doesn't work. When trying a few times, found out that it misses the step to install kuberay operator.

Related issue number

Checks

NA for most as this is only adjusting a doc.

• [ X ] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [ NA ] I've run scripts/format.sh to lint the changes in this PR.
• [ NA ] I've included any doc changes needed for https://docs.ray.io/en/master/.
  • [ NA ] I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• [ NA ] I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • [ NA ] Unit tests
  • [ NA ] Release tests
  • [ NA ] This PR is not tested :(

[kevin85421]

cc @andrewsykim would you mind reviewing this PR? Thanks!