fix #190 $ at end of TextBlock causes errors later

2a4189c7be7162622d9c9b3b252c9c3f5d5b843f should've done this:
https://github.com/mathquill/mathquill/commit/2a4189c7be7162622d9c9b3b252c9c3f5d5b843f#L0R492