fix(article): use locale formatted Modified (Last Updated) date

This applies the same fix as with the Published date format in commit
1da552c.

Fixes #613

Author: gertvdijk

templates/_includes/last_updated.html

@@ -6,8 +6,7 @@
 {% set delta = (article.modified - article.date).days %}
 {% if delta > 0 %}
 <h4>Last Updated</h4>
-{% set day = article.modified.strftime('%d')|int %}
-<time datetime="{{ article.modified.isoformat() }}">{{ article.modified.strftime('%b') }} {{ day }} {{- article.modified.strftime(', %Y') }}</time>
+<time datetime="{{ article.modified.isoformat() }}">{{ article.locale_modified }}</time>
 {% endif %}
 
 {% elif article.modified %}