Fix role assignment api-ref docs
Fix role assignment api-ref docs

The current docs list the available query params wrongly.

[1] https://github.com/openstack/keystone/blob/c025cb3/keystone/assignment/controllers.py#L961-L964

Change-Id: Ia7ec0e9dfaa8a67226a01531ac82b6391a7f3d81

openstack/keystone,openstack/keystone,mahak/keystone,openstack/keystone,mahak/keystone,mahak/keystone