update MathJax url

They moved the url and broke my site. Apparently they were unable to set
a 301 redirect at the old address, so they broke every site using
MathJax over https instead. Great. I could self-host it, but that would
inhibit caching across sites. I would switch to KaTeX, but I do not
think it can read MathML, and I prefer MathML in my html, because it has
more semantic meaning than spans with classes. It might be worth the
switch though, MathJax is much heavier than I would like.