fix: update year in docs template (#917)

Updated the copyright year in the docs template to reflect the new year.

docs/templates/index.html

@@ -99,7 +99,7 @@
             {% endblock content %}
         </div>
         <footer>
-            ©2017-2019 — <a class="white" href="http://vincentprouillet.com">Vincent Prouillet</a> and <a class="white" href="https://github.com/getzola/zola/graphs/contributors">contributors</a>
+            ©2017-2020 — <a class="white" href="http://vincentprouillet.com">Vincent Prouillet</a> and <a class="white" href="https://github.com/getzola/zola/graphs/contributors">contributors</a>
         </footer>
 
         <script type="text/javascript" src="{{ get_url(path="elasticlunr.min.js") }}"></script>