DEBUGFS: Correct location of debugfs API documentation.

Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

fs/debugfs/file.c

@@ -9,7 +9,7 @@
  *	2 as published by the Free Software Foundation.
  *
  *  debugfs is for people to use instead of /proc or /sys.
- *  See Documentation/DocBook/kernel-api for more details.
+ *  See Documentation/DocBook/filesystems for more details.
  *
  */