Fix codepen examples on the docs (#2145)

Interactive codepen examples on the docs stopped working after upgrading mkdocs to 1.1.2.

Author: epicfaace

docs/main.js

@@ -2,7 +2,7 @@ $(function() {
 function escape(html) {
     return html.replace(/&/g, "&amp;").replace(/>/g, "&gt;").replace(/</g, "&lt;").replace(/"/g, "&quot;");
 }
-$(".jsx").each(function() {
+$(".language-jsx").each(function() {
     var text = escape($(this).text());
     if (text.indexOf("render") === -1) {
         return;
@@ -29,4 +29,4 @@ script.src = url;
 
 document.head.appendChild(script);
 
-});
+});