kernel-parameters.txt : watchdog.txt should be wdt.txt

Documentation/watchdog/watchdog.txt does not exist, it is Documentation/watchdog/wdt.txt

Signed-off-by: Gabriel Craciunescu <nix.or.die@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>