Use %(name)s instead of %s everywhere within $._().

Summary:
%s has been deprecated for a while now, but these slipped through.

One more file I fixed up %s for.

Test Plan:
In the khan-exercises rootdir ran
   python -cSimpleHTTPServer
and visited
   http://localhost:8000/exercises/factoring_difference_of_squares_1.html
and clicked on the hint button until there were no more hints, and
verified it all showed up correctly, without %s's.

I also ran 'make exercises' in khan-root, which might possibly verify
I didn't introduce any javascript errors.

Reviewers: john, alpert

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D2157