doc: fix formatting in cephfs_mirror dev doc

Signed-off-by: Jos Collin <[email protected]>
(cherry picked from commit 8280d33)

Author: joscollin

doc/dev/cephfs-mirroring.rst

@@ -165,7 +165,7 @@ To stop a mirroring directory snapshots use::
   $ ceph fs snapshot mirror remove <fs_name> <path>
 
 Only absolute directory paths are allowed. Also, paths are normalized by the mirroring
-module, therefore, `/a/b/../b` is equivalent to `/a/b`.
+module, therefore, `/a/b/../b` is equivalent to `/a/b`::
 
   $ mkdir -p /d0/d1/d2
   $ ceph fs snapshot mirror add cephfs /d0/d1/d2