update google/code-prettify load URL

The rawgit service was shutdown in 2019, and people are supposed to
migrate to cdn.jsdelivr.net.
https://groups.google.com/g/js-code-prettifier/c/IFHev_kLm30

include/jsguide.js

@@ -57,7 +57,7 @@ window.initStyleGuide = function(init) {
 
   // Call the pretty-printer after we've fixed up the code blocks.
   var pretty = document.createElement('script');
-  pretty.src = 'https://cdn.rawgit.com/google/code-prettify/master/loader/' +
-      'run_prettify.js';
+  pretty.src = 'https://cdn.jsdelivr.net/gh/google/code-prettify@master/' +
+      'loader/run_prettify.js';
   document.body.appendChild(pretty);
 }.bind(null, window.initStyleGuide);