Fix grammar mistakes in license file

Correct usage of "is" to "are", as "Assets and content" is plural rather than singular.

closes #7700

LICENSE

@@ -2,8 +2,9 @@
 This Source Code is subject to the terms of the GNU General Public License, v. 3.0.
 If a copy of the GPL was not distributed with this file, You can obtain one at http://www.gnu.org/licenses/gpl-3.0.txt
 
-* Assets and content designed for Mozilla BrowserQuest is licensed under CC-BY-SA 3.0:
+* Assets and content designed for Mozilla BrowserQuest are licensed under CC-BY-SA 3.0:
 http://creativecommons.org/licenses/by-sa/3.0/
 
-* Assets and content designed for HabitRPG is licensed under CC-BY-NC-SA 3.0:
-http://creativecommons.org/licenses/by-nc-sa/3.0/
+* Assets and content designed for HabitRPG are licensed under CC-BY-NC-SA 3.0:
+http://creativecommons.org/licenses/by-nc-sa/3.0/
+