Add example object storage entry to geo secondary config

What does this Merge Request do and why?

Adds example object storage config to the suggested gdk.yml for a geo secondary, as it requires configuring a non-standard port (so as to not collide with the primary).

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.