Fix typo in systemd service definition file

Change-Id: Ia9dac04ad508fbaafbed24b2137b09872a1ca53c
BUG: 1010352
Signed-off-by: Justin Clift <jclift@redhat.com>
Reviewed-on: http://review.gluster.org/5981
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>