doc: md: Fix a file name to md-fault.c in fault-injection.txt

drivers/md/faulty.c has been renamed to md-faulty.c after
following commit merged int to the main line.

  935fe098 .

But the file name in fault-injection.txt has not been changed.
Now the actual file name and document are in sync.
Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/fault-injection/fault-injection.txt

 Fault injection capabilities infrastructure
 ===========================================
 
-See also drivers/md/faulty.c and "every_nth" module option for scsi_debug.
+See also drivers/md/md-faulty.c and "every_nth" module option for scsi_debug.
 
 
 Available fault injection capabilities