docs: adm1177: fix a broken reference

This reference was missing the .rst extension. This would be OK
if it were using the :doc: directive. So, switch to it.

As a side effect, this will create cross-reference links at
html output.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8d37f465888656224855a21f5bb01edb1ca66cf3.1582361738.git.mchehab+huawei@kernel.org


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