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/lib/Kconfig.debug b/lib/Kconfig.debug
index e4587eb..b7882d4 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -286,7 +286,7 @@
 	  write to these files.
 
 	  For detailed documentation on the debugfs API, see
-	  Documentation/DocBook/filesystems.
+	  Documentation/filesystems/.
 
 	  If unsure, say N.