Since the file was modified, update copyright to 2020

jme3-core/src/main/java/com/jme3/app/Application.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009-2012 jMonkeyEngine
+ * Copyright (c) 2009-2020 jMonkeyEngine
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without