Backport PR #6395: update mathjax CDN url in nbconvert template

closes  #6392

IPython/nbconvert/templates/html/mathjax.tpl

@@ -1,23 +1,23 b''
 {%- macro mathjax() -%}
     <!-- Load mathjax -->
-    <script src="https://c328740.ssl.cf1.rackcdn.com/mathjax/latest/MathJax.js?config=TeX-AMS_HTML"></script>
+    <script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML"></script>
     <!-- MathJax configuration -->
     <script type="text/x-mathjax-config">
     MathJax.Hub.Config({
         tex2jax: {
             inlineMath: [ ['$','$'], ["\\(","\\)"] ],
             displayMath: [ ['$$','$$'], ["\\[","\\]"] ],
             processEscapes: true,
             processEnvironments: true
         },
         // Center justify equations in code and markdown cells. Elsewhere
         // we use CSS to left justify single line equations in code cells.
         displayAlign: 'center',
         "HTML-CSS": {
             styles: {'.MathJax_Display': {"margin": 0}},
             linebreaks: { automatic: true }
         }
     });
     </script>
     <!-- End of mathjax configuration -->
 {%- endmacro %}
  No newline at end of file