Documentation: Reflect the new location of the NMI watchdog info

Commit 9919cba7

 ("watchdog: Update documentation") moved the
NMI watchdog documentation from nmi_watchdog.txt to
lockup-watchdogs.txt. Update the index file accordingly.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Don Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/20121021120551.4656d99b@endymion.delvare


Signed-off-by: Ingo Molnar <mingo@kernel.org>