RPM spec file: Fixed the description for glusterfs

Previously description was saying glusterfs is a
clustered file-system which changed to ditsributed
file-system.

Change-Id: I3ee80024ba317e912890ceff2080b82ce111be95
Signed-off-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-on: http://review.gluster.org/8619
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>