Fix license in copyright file

debian/copyright

@@ -8,7 +8,7 @@ License: GPL-3+
  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 the Free Software Foundation; either
- version 2 of the License, or (at your option) any later
+ version 3 of the License, or (at your option) any later
  version.
  .
  This program is distributed in the hope that it will be
@@ -24,4 +24,4 @@ License: GPL-3+
  .
  On Debian systems, the full text of the GNU General Public
  License version 2 can be found in the file
- `/usr/share/common-licenses/GPL-2'.
+ `/usr/share/common-licenses/GPL-3'.