Fix Geo guide for registry replication

What does this Merge Request do and why?

The old instruction simply didn't work.

host.docker.internal - now works for both Linux and Mac OS.

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.