input: docs: properly format ToC headings

"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: Dmitry Torokhov <[e-mail]>
Cc: [e-mail]
Signed-off-by: Vegard Nossum <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/input/input_kapi.rst

···
 Linux Input Subsystem kernel API
 ################################
 
-.. class:: toc-title
-
-        Table of Contents
-
 .. toctree::
+   :caption: Table of Contents
    :maxdepth: 2
    :numbered:
 

Documentation/input/input_uapi.rst

···
 Linux Input Subsystem userspace API
 ###################################
 
-.. class:: toc-title
-
-        Table of Contents
-
 .. toctree::
+   :caption: Table of Contents
    :maxdepth: 2
    :numbered:
 

Documentation/input/joydev/index.rst

···
 
 :Copyright: |copy| 1996-2000 Vojtech Pavlik <[e-mail]> - Sponsored by SuSE
 
-.. class:: toc-title
-
-	Table of Contents
-
 .. toctree::
+	:caption: Table of Contents
 	:maxdepth: 3
 
 	joystick