Fix arguments to handlers completely wrong #ImAnIdiot

Bug intro'd by #349 "Fix configuring handlers on inner math fields": https://github.com/mathquill/mathquill/commit/b8f191482b7d310a80c69b6eebacdff85db8eb12#commitcomment-8465453

Whoopsie! Hey @jwmerrill, can you sanity-check and merge this for me too?