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>
> Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>

Change-Id: Id2de0e353d3307023da9239f6dee8b59e8eb0d8f
BUG: 1175645
Reviewed-on: http://review.gluster.org/9295
Reviewed-by: Sachin Pandit <spandit@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Raghavendra Bhat <raghavendra@redhat.com>