[1.20.1] Correct invalid accessTransformers entry in mods.toml

[Fealtous]

Although it is not yet merged, over at Forge we're adding support for multiple AT files. The neo format for ATs is not the same and should not have been backported in this manner, however I implemented a fallback in case of mods having what yours has, so it won't crash if an end user uses the latest build.

This PR corrects the toml to match our upcoming format and still remains compatible with all prior forge versions, as we did not originally have a toml entry for this.
See MinecraftForge/MinecraftForge#10786 for any additional information.

[LostLuma]

I'd guess it should also be moved to the top, instead of being inside one of the [[mixins]] blocks?

Either way though, I can change this next time I make a release, but will probably not merge this as I usually just force push those branches with the backport commit.

[Fealtous]

We actually don't have a [[mixins]] block at all in the spec, so that block isn't actually doing anything either. But yeah not a huge rush since there's a fallback in case of mods having [[accessTransformers]], just whenever is most convenient.