docs: hwmon: remove the extension from .rst files

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: Guenter Roeck <linux@roeck-us.net>