'group_id' is mentioned as an optional parameter in api-ref doc for
POST /v3/{project_id}/group_snapshots API but in the code if we don't
pass group-id it returns BadRequest.
This patch fixes this discrepancy by updating the api-ref according
to the code.
Closes-Bug: #1735177
Change-Id: Id2165a07995ab706d4a327f7340dccee7799c7b6