zsync: change license to artistic2

Found in the overview section on http://zsync.moria.org.uk/

pkgs/tools/compression/zsync/default.nix

···
   meta = with lib; {
     homepage = "http://zsync.moria.org.uk/";
     description = "File distribution system using the rsync algorithm";
-    license = licenses.free;
+    license = licenses.artistic2;
     maintainers = with maintainers; [ viric ];
     platforms = with platforms; all;
   };