Fix overlapping radio buttons on mobile

Summary:
The span which held the text of radio buttons wasn't styled
correctly, so it would sometimes overlap with the circle bit. This fixes
that.

Test Plan:
 - Load an exercise with lots of text in one of the multiple choice
   choices.
   - See that the text wraps correctly.

Reviewers: aria

Reviewed By: aria

Differential Revision: https://phabricator.khanacademy.org/D15191