Fix issue with code examples not working after any demos

This works around an issue where a demo embedded within the page would cause any code samples in the rest of the page to not be highlighted correctly. Prism would try to format the demo HTML as CLI code and throw an error in the process.

demo_tpl.js

@@ -1,3 +1,7 @@
+// Note that the space in the <code> </code> blocks is significant:
+// without it, any demo in a page would break the rest of the code
+// examples throughout the page.
+
 module.exports = `
 	<ul>
 		<li class="tab" data-tab="demo">Demo</li>
@@ -8,9 +12,9 @@ module.exports = `
 		<iframe></iframe>
 	</div>
 	<div class="tab-content" data-for="html">
-		<pre class="line-numbers language-html"><code></code></pre>
+		<pre class="line-numbers language-html"><code> </code></pre>
 	</div>
 	<div class="tab-content" data-for="js">
-		<pre class="line-numbers language-js"><code></code></pre>
+		<pre class="line-numbers language-js"><code> </code></pre>
 	</div>
 `;