Changes stimpack negative chemical to synaptizine

After a liberal amount of butthurt this was suggested as a better 'negative' chemical for the stimpacks.

Synaptizine gives you toxin damage, meaning you can't abuse them, but also helps in combat to reduce stuns (such as from the goliath. Additionally, it's only two units and works better than oxyloss in this case while fitting the 'unstable starcraft/fallout stimpack' theme more.