Clarify Copyright year

Based on http://copyright.gov/title17/92chap4.html#401 and being more consistent with the rest of the document.

See also https://make.wordpress.org/themes/2014/07/08/proper-copyrightlicense-attribution-for-themes/

Fixes #35278


git-svn-id: https://develop.svn.wordpress.org/trunk@36855 602fd350-edb4-49c9-b593-d223f7449a82

Author: aaronjorbin

src/license.txt

@@ -1,6 +1,6 @@
 WordPress - Web publishing software
 
-Copyright 2016 by the contributors
+Copyright 2011-2016 by the contributors
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by