debian/copyright: Point to GPL-2 instead of GPL.

debian/copyright

@@ -24,4 +24,4 @@ License:
    02110-1301, USA.
 
 On Debian GNU/Linux systems, the complete text of the GNU General
-Public License can be found in `/usr/share/common-licenses/GPL'.
+Public License version 2 can be found in `/usr/share/common-licenses/GPL-2'.