rbd: correct sysfs snap attribute documentation

Each attribute is prefixed with "snap_".
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>

Documentation/ABI/testing/sysfs-bus-rbd

@@ -65,11 +65,11 @@ snap_*
 Entries under /sys/bus/rbd/devices/<dev-id>/snap_<snap-name>
 -------------------------------------------------------------
 
-id
+snap_id
 
 	The rados internal snapshot id assigned for this snapshot
 
-size
+snap_size
 
 	The size of the image when this snapshot was taken.