Fix wrong description for nova image-create
Fix wrong description for nova image-create

In the subsection "Create an image (nova)", there is a sentence said "You
cannot create a snapshot from an instance that has an attached volume",
which must be wrong. Currently, nova supports creating image from an
instance with attached volumes.

Change-Id: I811a845b8ee028ab83a12af03364930a0394e18d
Closes-Bug: #1341348

AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,Akasurde/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual