Removed polyfill.io link in mkdocs.yml

https://blog.cloudflare.com/automatically-replacing-polyfill-io-links-with-cloudflares-mirror-for-a-safer-internet

Fixes #14

Author: Progi1984

mkdocs.yml

@@ -22,7 +22,7 @@ extra:
   generator: false
 extra_javascript:
   - assets/mathjax.js
-  - https://polyfill.io/v3/polyfill.min.js?features=es6
+  - https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?version=3.111.0&features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
 markdown_extensions:
   ## Syntax highlighting
@@ -61,4 +61,4 @@ nav:
   - Developers:
     - 'Coveralls': 'https://coveralls.io/github/PHPOffice/Math'
     - 'Code Coverage': 'coverage/index.html'
-    - 'PHPDoc': 'docs/index.html'
+    - 'PHPDoc': 'docs/index.html'