ox-md.el: Fix a library keyword in the comment section

* ox-md.el: Fix a library keyword in the comment section.

lisp/ox-md.el

@@ -3,7 +3,7 @@
 ;; Copyright (C) 2012, 2013  Free Software Foundation, Inc.
 
 ;; Author: Nicolas Goaziou <n.goaziou@gmail.com>
-;; Keywords: org, wp, tex
+;; Keywords: org, wp, markdown
 
 ;; GNU Emacs is free software: you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by