Remove note about updating gitlab-shell bin

What does this Merge Request do?

Removes note about gitlab-shell ruby bin for Mac users as it's no longer a ruby script. It's now a Go binary.

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.