media: admin-guide: properly format ToC heading

"class:: toc-title" was a workaround for older Sphinx versions that are
no longer supported.

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-4-vegard.nossum@oracle.com

Documentation/admin-guide/media/index.rst

@@ -20,16 +20,8 @@ Documentation/driver-api/media/index.rst
   - for driver development information and Kernel APIs used by
     media devices;
 
-The media subsystem
-===================
-
-.. only:: html
-
-    .. class:: toc-title
-
-        Table of Contents
-
 .. toctree::
+	:caption: Table of Contents
 	:maxdepth: 2
 	:numbered: