docs(rxjs.dex): replace polyfill.io with a Cloudflare equivalent (#7487)

Author: tnfssc

apps/rxjs.dev/src/index.html

@@ -84,7 +84,7 @@
   <script>
     if (!Object.assign) {
       // polyfill other non-evergreen browsers in a blocking way
-      var polyfillUrl = "https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.find&flags=gated&unknown=polyfill";
+      var polyfillUrl = "https://cdnjs.cloudflare.com/polyfill/v2/polyfill.min.js?features=default,Array.prototype.find&flags=gated&unknown=polyfill";
 
       // send a blocking XHR to fetch the polyfill
       // then append it to the document so that its eval-ed synchronously