fix a typo in Documentation/pi-futex.txt

Signed-off-by: Riccardo Magliocchetti <riccardo@datahost.it>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

Documentation/pi-futex.txt

@@ -118,4 +118,4 @@ properties of futexes, and all four combinations are possible: futex,
 robust-futex, PI-futex, robust+PI-futex.
 
 More details about priority inheritance can be found in
-Documentation/rtmutex.txt.
+Documentation/rt-mutex.txt.