Documentation: fix tty link

Documentation: fix tty link

The commit below causes the following warnings:
Documentation/driver-api/index.rst:14: WARNING: toctree contains reference to nonexisting document 'driver-api/tty'
Documentation/driver-api/tty/index.rst: WARNING: document isn't included in any toctree

This is because I omitted the "/index" part from the link. That part is
needed for directories and I was confused by the surrounding links which
are links to files. Fix it in this patch.

Fixes: b96cd8b05ead ("Documentation: move tty to driver-api")
Cc: Jonathan Corbet <corbet@lwn.net>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220419074302.29632-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>