[pkg] resolve lintian warning: invalid-short-name-in-dep5-copyright

The name of the GPLv3 license in debian/copyright has to match one the dep5
processor understands

debian/copyright

@@ -4,7 +4,7 @@ Source: <https://0xacab.org/leap/riseup_vpn>
 
 Files: *
 Copyright: 2018 LEAP Encryption Access Project <info@leap.se>
-License: GPLv3
+License: GPL-3+
  Permission is hereby granted, free of charge, to any person obtaining a
  copy of this software and associated documentation files (the "Software"),
  to deal in the Software without restriction, including without limitation