cli/snapshot : update of a snapshot delete syntax in documentation

Change-Id: Id1a4b9684a8dd5750ee6eed841e3d5195407fb7e
BUG: 1112613
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8534
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>