Relative URL issues with the related post examples

I noted that if you set the `ARTICLE_URL` site variable to have some depth and just create relative links, they will not link correctly.

by prefacing the link with `{{ SITEURL}}/` it seems to ensure proper links are created and works with the `make devserver` mode

docs/plugins.rst

@@ -302,7 +302,7 @@ For example::
     {% if article.related_posts %}
         <ul>
         {% for related_post in article.related_posts %}
-            <li><a href="{{ related_post.url }}">{{ related_post.title }}</a></li>
+            <li><a href="{{ SITEURL }}/{{ related_post.url }}">{{ related_post.title }}</a></li>
         {% endfor %}
         </ul>
     {% endif %}