Fixed Toolbar plugin example (#1726)

There was a function parameter missing in one of the Toolbar plugin examples.

plugins/toolbar/index.html

@@ -59,7 +59,7 @@ <h1>How to use</h1>
 	<p>If you need more control, you can provide a function to <code>registerButton</code> that returns either a <code>span</code>, <code>a</code>, or
 		<code>button</code> element.</p>
 
-	<pre><code class="language-javascript">Prism.plugins.toolbar.registerButton('select-code', function() {
+	<pre><code class="language-javascript">Prism.plugins.toolbar.registerButton('select-code', function(env) {
 	var button = document.createElement('button');
 	button.innerHTML = 'Select Code';
 
@@ -131,4 +131,4 @@ <h1>How to use</h1>
 <script src="code.js"></script>
 
 </body>
-</html>
+</html>