Add integrity to mathjax

Author: rossabaker

_includes/_head.html

@@ -25,7 +25,7 @@
     };
   </script>
   <script type="text/javascript" id="MathJax-script" async
-    src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js">
+    src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.2.2/es5/tex-mml-chtml.js" crossorigin="anonymous" integrity="sha384-Wuix6BuhrWbjDBs24bXrjf4ZQ5aFeFWBuKkFekO2t8xFU0iNaLQfp2K6/1Nxveei">
   </script>
   {% endif %}
 </head>