Fix the default MathJax v2 script snippet

[mortenpi]

The MathJax website suggests using the following URL if you want to load MathJax v2 in a <script> tag:

https://cdn.jsdelivr.net/npm/mathjax@2/MathJax.js?config=TeX-AMS-MML_CHTML

But that looks to be broken -- in the browser, MathJax fails to load with:

GET https://cdn.jsdelivr.net/npm/mathjax@2/config/TeX-AMS-MML_CHTML.js?V=2.7.9 net::ERR_ABORTED 404

As far as I can tell, it's because there is no such .js file under config/ for 2.7.9.

Unless it's a CDN problem, I am guessing a different ?config should be used. Here I suggest switching it to ?config=TeX-AMS_CHTML, which does load properly, although I do not really have an opinion on whether that's the configuration that should be recommended as the default.