Fix typo in Balancing Chemical Equations.

Summary:
Mg is +2, and (OH) is -1. The subscript 2 should be outside of the brackets for
(OH).  Thanks to Carl Shan, Chloe Lim, Hayley Chen, and Yiju Tuan for finding
the typo.

Test Plan:
Opened the exercise locally and observed that it rendered correctly, including
hints.

Reviewers: tom, eater

Reviewed By: eater

CC: eater

Differential Revision: http://phabricator.khanacademy.org/D1023