I don't think there is a mounting point specifically for the earth lead . Just take it to the central mounting bolt for the rectifier, which is nearby. It might be an idea to lengthen the lead and put a snap connector in the middle so that you can disconnect both the earth lead and the red live lead by pulling the snap connectors apart when removing the battery. That saves having to fiddle around unscrewing terminal connections with the battery still attached to the bike - plenty of scope there for frustration and dropped/lost screws.
Graham