[media] media-ioc-g-topology.rst: fix a c domain reference

One reference there is still using :ref:. Fix it, to solve this
warning:
  Documentation/media/uapi/mediactl/media-ioc-g-topology.rst:236: WARNING: undefined label: media-v2-intf-devnode (if the link has no caption the label must precede a section header)

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>