Update single-file example

static/html/single-file-example.html

@@ -13,10 +13,13 @@
     <div id="root"></div>
     <script type="text/babel">
     
-      ReactDOM.render(
-        <h1>Hello, world!</h1>,
-        document.getElementById('root')
-      );
+      function MyApp() {
+        return <h1>Hello, world!</h1>;
+      }
+
+      const container = document.getElementById('root');
+      const root = ReactDOM.createRoot(container);
+      root.render(<MyApp />);
 
     </script>
     <!--