Add Greek letters to learner docs for math expression problems

Currently, we just spell out the names of allowed Greek letters because I didn't know of a way to make Greek letters in Sphinx. An engineer sent me this:

You should be able to get greek letters with this: http://docutils.sourceforge.net/docs/ref/rst/directives.html#unicode-character-codes

It would be great to include the Greek letters themselves in the docs.

http://www.rapidtables.com/math/symbols/greek_alphabet.htm