Remove nested "group" wrapper from Create custom group API example.
All properties (name, display_name, source, allow_reference, user_ids)
are now correctly shown at the top level of the payload.
Fixes#33554
Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: Carrie Warner (Mattermost) <cwarnermm@users.noreply.github.com>