media: userspace-api: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/glossary.rst

@@ -116,7 +116,7 @@ Glossary
 	  - :term:`RC API`; and
 	  - :term:`V4L2 API`.
 
-	See :doc:`index`.
+	See Documentation/userspace-api/media/index.rst.
 
     MC API
 	**Media Controller API**