Update docs links for the CDN

Now that cdnjs is mirroring the beta files, we know what the final
locations are going to be. These identifiers will need to be updated
during the release process, just like all of the others.

This closes https://github.com/select2/select2/issues/2955.

docs/index.html

@@ -21,7 +21,7 @@ slug: home
           </a>
         </p>
         <p>
-          <strong>Version</strong> 4.0.0 <em>beta 2</em>
+          <strong>Version</strong> 4.0.0<em>-beta.2</em>
         </p>
       </div>
     </div>
@@ -70,8 +70,8 @@ slug: home
         section of your HTML.
 
 <pre class="code">
-&lt;link href="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/select2.min.css" /&gt;
-&lt;script src="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/select2.min.js"&gt;&lt;/script&gt;
+&lt;link href="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0-beta.2/css/select2.min.css" /&gt;
+&lt;script src="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0-beta.2/js/select2.min.js"&gt;&lt;/script&gt;
 </pre>
 
         <div class="alert alert-info">