fs: update location of filesystems documentation

The filesystem documentation was moved from DocBook to
Documentation/filesystems/. Update it at the sources.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 354e2ab..6dabc4a 100644
--- a/fs/debugfs/file.c
+++ b/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/filesystems for more details.
+ *  See Documentation/filesystems/ for more details.
  *
  */