docs: watchdog: fix obsolete include file reference in pcwd

The file linux/pcwd.h is not more present in the kernel
sources. Its information is now moved into the file:

include/uapi/linux/watchdog.h
Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Link: https://lore.kernel.org/r/20210223111324.309285-1-f.suligoi@asem.itSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/watchdog/pcwd-watchdog.rst

@@ -47,7 +47,7 @@ Documentation and Driver by Ken Hollis <kenji@bitgate.com>
 	WDIOC_GETSTATUS
 		This returns the status of the card, with the bits of
 		WDIOF_* bitwise-anded into the value.  (The comments
-		are in linux/pcwd.h)
+		are in include/uapi/linux/watchdog.h)
 
 	WDIOC_GETBOOTSTATUS
 		This returns the status of the card that was reported