Make the help button from title screen load core units again.

Just a quick fix to the recent no units bug.

Note that clicking on this button in titlescreen will unload
previously loaded campaign or MP. But this also guarantee that the
main help is only about core.

src/game.cpp

@@ -2109,6 +2109,7 @@ static int play_game(int argc, char** argv)
 			about::show_about(game.disp());
 			continue;
 		} else if(res == gui::SHOW_HELP) {
+			game.reset_game_cfg();
 			help::help_manager help_manager(&game.game_config(), &game.units_data(), NULL);
 			help::show_help(game.disp());
 			continue;