Update command to install google cloud sdk

What does this Merge Request do and why?

I've changed the command that installs google cloud sdk to the more recent convention for brew. I've also moved a note up in the instructions to preempt users who may have a problem by trying to run gcloud before adding it to their path.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.