Give Twitch ult the same scaling as Vayne's

Up it to 20 ad per point like Vayne's. It currently increases by 8 which is much too little considering his low base ad. For one of the most vulnerable adcs in the game (no hard cc or hard mobility), he's just not as threatening damage-wise as he should be.

This will also help him stand up to Vayne and act as an indirect nerf to her by giving her a dangerous new lane opponent.