Updated index.html to fix broken instructions

Requiring components without the "index.js" part and calling "loadLanguages" results in a function not found error for me. Adding index.js, as commented by @Golmote, fixes that error.

Ref: PrismJS#972

index.html

@@ -188,7 +188,7 @@ <h2>Usage with Node</h2>
 	<p>Example:</p>
 
 	<pre><code class="language-js">var Prism = require('prismjs');
-var loadLanguages = require('prismjs/components');
+var loadLanguages = require('prismjs/components/index.js');
 loadLanguages(['haml']);
 
 // The code snippet you want to highlight, as a string