mgr/volumes: fix error message

... when trying to remove a non-existent fs subvolume group
snapshot.

Fixes: http://tracker.ceph.com/issues/40014
Signed-off-by: Ramana Raja <rraja@redhat.com>
(cherry picked from commit 834bd72)

src/pybind/mgr/volumes/module.py

@@ -538,7 +538,7 @@ def _cmd_fs_subvolumegroup_snapshot_rm(self, inbuf, cmd):
                     return 0, "", ""
                 else:
                     return -errno.ENOENT, "", \
-                           "Subvolume group snapshot '{0}' not found, cannot remove it".format(sub_name)
+                           "Subvolume group snapshot '{0}' not found, cannot remove it".format(snap_name)
 
         return 0, "", ""