Change class "page-date" to "post-date"

_layouts/post.html

@@ -4,7 +4,7 @@ layout: default
 
 <article class="post">
   <h1 class="post-title">{{ page.title }}</h1>
-  <time datetime="{{ page.date | date_to_xmlschema }}" class="page-date">{{ page.date | date_to_string }}</time>
+  <time datetime="{{ page.date | date_to_xmlschema }}" class="post-date">{{ page.date | date_to_string }}</time>
   {{ content }}
 </article>