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 <[e-mail]>
Acked-by: Guenter Roeck <[e-mail]>
Link: https://lore.kernel.org/r/3d6437[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/hwmon/adm1266.rst

···
 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