Fix not showing DEG/RAD

Summary:
The issue was really using the wrong CSS selector, which is obvious if
you look at what the two selectors represent:

{F15388}
{F15387}

Test Plan:
* Subperseus
* Go to Sample and population standard deviation
* Calculator should have deg/rad button! How RAD is that?

Reviewers: alan, jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D7657