Update mkdocs.yml

Updating URL as advised

Author: hsutaiyu

mkdocs.yml

@@ -658,7 +658,7 @@ validation:
 
 extra_javascript:
   - _site_essentials/js/mathjax.js
-  - https://cdnjs.cloudflare.com/polyfill/
+  - https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
   - https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.16.7/katex.min.js  
   - https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.16.7/contrib/auto-render.min.js