Fix some errors in 'dashboard_manage_volumes.rst'
Fix some errors in 'dashboard_manage_volumes.rst'

The "Snapshot" is not one item of the "Volume Source", and the
"Availability Zone" is a independent item and not under
"Volume Source". This patch fix them.

Change-Id: I40d6735a2e3faf856b208d9ca474dad1304bd901
Closes-bug: #1504475

AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals