Update mkdocs.yml

Author: collijk

mkdocs.yml

@@ -73,5 +73,5 @@ extra_javascript:
   - https://unpkg.com/tablesort@5.3.0/dist/tablesort.min.js
   - _javascript/tablesort.js
   - _javascript/mathjax.js
-  - https://polyfill.io/v3/polyfill.min.js?features=es6
+  - https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js