Use correct heading tag for date subtitles

website/blog/templates/blog/blog_post_list_page.html

@@ -11,11 +11,11 @@
   <section class="container">
     {% for page in listing_pages %}
       {% ifchanged %}
-        <h3 id="date-{{ page.date|date:'Y-m' }}" class="date-header">
+        <h2 id="date-{{ page.date|date:'Y-m' }}" class="date-header">
           <time datetime="{{ page.date|date:'Y-m' }}" title="{{ page.date|date:'F Y' }}">
             {{ page.date|date:"Y-m" }}
           </time>
-        </h3>
+        </h2>
       {% endifchanged %}
 
       {% include "common/listing-item.html" %}