Update post.html

_layouts/post.html

@@ -10,7 +10,7 @@ section_id: blog
     {% if page.header %}
       <div class="post-header" style="background-image: url({{site.url}}/{{page.header[0]}})">
       </div>
-    else
+    {% else %}
       <div class="two spacing"> 
     </div>
     {% endif %}