doc: updated the copyright years in markdown as well.

Author: agentzh

README.markdown

@@ -995,7 +995,7 @@ This module is licensed under the BSD license.
 
 Copyright (C) 2009-2017, by Xiaozhe Wang (chaoslawful) <chaoslawful@gmail.com>.
 
-Copyright (C) 2009-2017, by Yichun "agentzh" Zhang (章亦春) <agentzh@gmail.com>, OpenResty Inc.
+Copyright (C) 2009-2018, by Yichun "agentzh" Zhang (章亦春) <agentzh@gmail.com>, OpenResty Inc.
 
 All rights reserved.