Use updated mathjax provided by jsdelivr. (#215)

layouts/partials/posts/math.html

@@ -1,6 +1,6 @@
 {{- if .Params.math -}}
   <script type="text/javascript" async
-    src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.4/MathJax.js?config=TeX-AMS-MML_HTMLorMML-full">
+    src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">
     MathJax.Hub.Config({
       tex2jax: {
         inlineMath: [['$','$']],