Fix readthedocs domain

index.html

@@ -60,7 +60,7 @@
 <h3>
 <a id="documentation" class="anchor" href="#documentation" aria-hidden="true"><span class="octicon octicon-link"></span></a>Documentation</h3>
 
-<p>You can find a lot of documentation, including a 'Getting started' section here: <a href="http://solarium.readthedocs.org/en/stable/">http://solarium.readthedocs.org/en/stable/</a></p>
+<p>You can find a lot of documentation, including a 'Getting started' section here: <a href="http://solarium.readthedocs.io/en/stable/">http://solarium.readthedocs.org/en/stable/</a></p>
 
 <h3>
 <a id="contributing" class="anchor" href="#contributing" aria-hidden="true"><span class="octicon octicon-link"></span></a>Contributing</h3>