Update MathJax implementation to include required polyfill

[ErroneousBosch]

Explain the problem.
The current embed of MathJax 3 for standalone HTML does not work properly for rendering. This is because per the MathJax "getting started", they also require a polyfill: https://www.mathjax.org/#gettingstarted

The correct script tags should be:

<script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
<script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>

Pandoc version?
3.1

[jgm]

Thanks for pointing that out!