Replace polyfill.io with Cloudflare Image

Signed-off-by: Lei Hsiung <leihsiung.ray@gmail.com>

index.html

@@ -42,7 +42,7 @@
   <link rel="stylesheet" href="https://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
   <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
   <script src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
-  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
+  <script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6"></script>
   <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>
   <script>
   $( function() {