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