docs: hwmon: adm1266.rst: fix a broken reference

The reference was missing the extension, causing the
check script to complain.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/3d64372dabcdcea144cdc9972c245812ea2a84cb.1603791716.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>