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

Documentation/hwmon/adm1266.rst

@@ -20,7 +20,7 @@ ADM1266 is a sequencer that features voltage readback from 17 channels via an
 integrated 12 bit SAR ADC, accessed using a PMBus interface.
 
 The driver is a client driver to the core PMBus driver. Please see
-Documentation/hwmon/pmbus for details on PMBus client drivers.
+Documentation/hwmon/pmbus.rst for details on PMBus client drivers.
 
 
 Sysfs entries