Doc: pps: Fix file name in pps.txt

This patch fix a file name of example code.
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Acked-by: NRodolfo Giometti <giometti@enneenne.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/pps/pps.txt

@@ -125,7 +125,7 @@ The same function may also run the defined echo function
 (pps_ktimer_echo(), passing to it the "ptr" pointer) if the user
 asked for that... etc..
 
-Please see the file drivers/pps/clients/ktimer.c for example code.
+Please see the file drivers/pps/clients/pps-ktimer.c for example code.
 
 
 SYSFS support