Fix typos; MathML -> MathJAX


AFAIK MathML is a dinosaur that nobody uses, and MathJAX is the preferred mathematical typesetting technology for the web.  For ebooks I'm not sure what the best choice is, but I am sure that MathML isn't the answer.

If you disagree with my stance on MathJAX vs. MathML, but still want my spelling/grammar improvements, feel free to just pull the first of these two commits.