x86/watchdog: Fix Kconfig help text file path reference to lockup watchdog documentation

Signed-off-by: Benjamin Peterson <bp@benjamin.pe>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 9919cba7

 ("watchdog: Update documentation")
Link: http://lkml.kernel.org/r/20170521002016.13258-1-bp@benjamin.pe
Signed-off-by: Ingo Molnar <mingo@kernel.org>