Fix version number in footer.

docs/_includes/footer.html

@@ -8,7 +8,7 @@
       Maintained by <a href="https://github.com/kevin-brown">Kevin Brown</a> and <a href="https://github.com/ivaynberg">Igor Vaynberg</a>.
     </p>
     <ul class="s2-docs-footer-links">
-      <li>Currently v4.0.0-rc.2</li>
+      <li>Currently v4.0.0</li>
       <li><a href="https://github.com/select2/select2">GitHub</a></li>
       <li><a href="./examples.html">Examples</a></li>
       <li><a href="./options.html">Options</a></li>