media: index.rst: don't write "Contents:" on PDF output

Right now, Sphinx unconditionally creates a blank page with
just "Contents:" on it, on PDF output. While this makes sense
for html, it doesn't o PDF, as LaTeX does what's required
automatically.

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

Documentation/media/index.rst

 Linux Media Subsystem Documentation
 ===================================
 
-Contents:
+.. only:: html
+
+   .. class:: toc-title
+
+        Table of Contents
 
 .. toctree::
    :maxdepth: 2