locking/Documentation: Update code path

lockdep code has been moved from kernel/ to kernel/locking/ by commit
8eddac3f

 ("locking: Move the lockdep
code to kernel/locking/"). But, path to lockdep code in document was not
updated.

This commit updates the path.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1421176921-27688-1-git-send-email-sj38.park@gmail.com
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>