[docs] Use current date in copyright footer

This will be based on the site generation time, making doc generation
slightly less deterministic but that's ok. Now we won't depend on
helpful community members updating it for us (#2874) when we forget,
it'll just happen naturally the next time the site is generated.

_layouts/default.html

@@ -79,7 +79,7 @@
         <a href="/react/acknowledgements.html">Acknowledgements</a>
       </div>
       <div class="right">
-        &copy; 2013&ndash;2015 Facebook Inc.<br>
+        &copy; 2013&ndash;{{ site.time | date: '%Y' }} Facebook Inc.<br>
         Documentation licensed under <a href="http://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a>.
       </div>
     </footer>