[ski ci] Update post-header.html, last modified with | date_to_string

web/_includes/post-header.html

 <div>
-<p class="text-muted">{{ page.date | date_to_string }}, <small>last modified at {{ page.last_modified_at }}</small></p>
+<p class="text-muted">{{ page.date | date_to_string }}, <small>last modified at {{ page.last_modified_at | date_to_string }}</small></p>
 </div>
\ No newline at end of file