Fix Liquid syntax error

Author: mattt

_layouts/yearly_archive.html

@@ -5,25 +5,25 @@
           <header>
             <h2>Posts in <strong>{{ page.date | date: "%Y" }}</strong></h2>
           </header>
-          
+
           <article class="content">
             <ul class="postlist">
               {% assign month = "" %}
               {% for post in page.posts %}
-            
-                {% assign post_month = post.date || date: "%B" %}
+
+                {% assign post_month = post.date | date: "%B" %}
                 {% if post_month != month %}
-            
+
                   {% assign month = post_month %}
                 <h2>{{ post_month }}</h2>
-            
+
                 {% endif %}
-            
+
                 {% include post-item.html %}
-            
+
               {% endfor %}
             </ul>
-            
+
             <br />
             <ul class="pager">
               {% if page.previousArchive %}