add mathjax script URL to custom.html

Author: reshamas

_includes/head/custom.html

@@ -7,3 +7,6 @@
 <meta name="msapplication-TileColor" content="#da532c">
 <meta name="msapplication-config" content="/assets/images/browserconfig.xml">
 <meta name="theme-color" content="#ffffff">
+<script type="text/javascript" id="MathJax-script" async
+  src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">
+</script>