[doc] Correct the wrong 'System' tab
[doc] Correct the wrong 'System' tab

Now the 'Volume Types' and 'Volumes' are under 'Volume' tab
of the 'Admin' tab, this patch fix them.

Change-Id: I9242c083aab591fa1aca58dfccc69fbf0d320ab8

noironetworks/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,openstack/horizon,openstack/horizon,noironetworks/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,noironetworks/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,openstack/horizon,ChameleonCloud/horizon,openstack/horizon,noironetworks/horizon