Fixed alias example in extending page (#2011)

This fixes the example on how to add a language alias to a language definition.

extending.html

@@ -177,7 +177,7 @@ <h1>Creating a new language definition</h1>
 
 			<p>To add the alias, we add this line at the end of <code>prism-foo-bar.js</code>:</p>
 
-			<pre><code class="language-js">Prism.languages.foo = Prism.language['foo-bar'];</code></pre>
+			<pre><code class="language-js">Prism.languages.foo = Prism.languages['foo-bar'];</code></pre>
 
 			<p>Aliases also have to be registered in <code>components.json</code> by adding the <code>alias</code> property to the language entry. In this example, the updated entry will look like this:</p>