[media] vidioc-enumstd.rst: fix a broken reference

Somehow, the conversion broke a reference here. Re-add it.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/vidioc-enumstd.rst

@@ -269,7 +269,7 @@ support digital TV. See also the Linux DVB API at
 
 .. _video-standards:
 
-.. flat-table:: Video Standards (based on [])
+.. flat-table:: Video Standards (based on :ref:`itu470`)
     :header-rows:  1
     :stub-columns: 0