Fix documentation error
Fix documentation error

There was a mistake in an earlier patch where
'snapshot' was used instead of 'volume'.  This
follow-up patch corrects the wording.

Change-Id: Ib1a03521c277c12d1b7f1c91eb1f2df1972e883f

openstack/cinder,mahak/cinder,openstack/cinder,mahak/cinder