Update article.html (#39)

The cover image wasn't showing properly because of a <span> element that was not closed.
Thanks for the great theme!

templates/article.html

@@ -82,7 +82,7 @@
         <h1 class="post-title">{{ article.title }}</h1>
         <!-- TODO : Proper class for headline -->
         {% if article.headline %}
-            <span class="blog-description">{{ article.headline }}</h3>
+            <span class="blog-description">{{ article.headline }}</span>
         {% endif %}
         <span class="post-meta">
             {% for author in article.authors %}