On almost all places, we're including ReST files without the
extension.

Let's remove the extension here as well, in order to use just
one standard.
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>