dm: add dm-clone to the documentation index

Fixes: 7431b783 ("dm: add clone target")
Signed-off-by: NDiego Calleja <diegocg@gmail.com>
Signed-off-by: NNikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

Documentation/admin-guide/device-mapper/index.rst

@@ -8,6 +8,7 @@ Device Mapper
     cache-policies
     cache
     delay
+    dm-clone
     dm-crypt
     dm-flakey
     dm-init