uss/gluster: Fix typo error in the description for USS under "gluster volume
set help"

gluster volume set help for uss shows "User Servicable Snapshots"
whereas it should be "User Serviceable Snapshots"

Change-Id: I3cc8b3ea2cb6d209e1a12678eb7d0e68f4160d99
BUG: 1160236
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/9041
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>