jgmenu: update license

pkgs/applications/misc/jgmenu/default.nix

···
   meta = with lib; {
     homepage = "https://github.com/johanmalm/jgmenu";
     description = "Small X11 menu intended to be used with openbox and tint2";
-    license = licenses.gpl2;
+    license = licenses.gpl2Plus;
     platforms = platforms.linux;
     maintainers = [ maintainers.romildo ];
   };