doc: misc-device: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Julien Panis <jpanis@baylibre.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-9-vegard.nossum@oracle.com

Documentation/misc-devices/index.rst

@@ -7,11 +7,8 @@ Assorted Miscellaneous Devices Documentation
 This documentation contains information for assorted devices that do not
 fit into other categories.
 
-.. class:: toc-title
-
-	   Table of contents
-
 .. toctree::
+   :caption: Table of contents
    :maxdepth: 2
 
    ad525x_dpot