Documentation: fix locking rt-mutex doc refs

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/pi-futex.txt b/Documentation/pi-futex.txt
index aafddbe..b154f6c 100644
--- a/Documentation/pi-futex.txt
+++ b/Documentation/pi-futex.txt
@@ -119,4 +119,4 @@
 robust-futex, PI-futex, robust+PI-futex.
 
 More details about priority inheritance can be found in
-Documentation/rt-mutex.txt.
+Documentation/locking/rt-mutex.txt.