Remove unnecessary "admin" from cinder type-create

cinder command doesn't contain "admin" subcommand, but there is
a typo in the doc. Maybe the typo came from --os-username which
was removed when writting the doc.
In this doc, there is not any --os-username descriptions. So this
patch removes "admin" instead of adding --os-username.

Change-Id: I75ce6ae51f1dd7a17d37076a94fcd2c9778cb806
Merge "Remove unnecessary "admin" from cinder type-create"