Documentation/watchdog/hpwdt: Fix Reference

The IPMI documentation moved to Documentation/driver-api/ipmi.rst.
Update reference to reflect new location.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230210184247.221134-2-jerry.hoemann@hpe.com


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