Update post-header-home.html

web/_includes/post-header-home.html

 <div>
-<p><span style="float:left;">{{ post.date | date_to_string }}</span><span style="float:right;"> <small>last modified at {{ page.last_modified_at | date_to_string }}</small></span></p>
+<p>{{ post.date | date_to_string }}, <small>last update {{ page.last_modified_at | date_to_string }}</small></p>
 </div>
\ No newline at end of file