Fix theme for new pelican version

pelican-themes/pelican-bootstrap3/templates/base.html

@@ -92,7 +92,7 @@
     {% endif %}
 
     {% if category and CATEGORY_FEED_ATOM %}
-        <link href="{{ SITEURL }}/{{ CATEGORY_FEED_ATOM|format(category.slug) }}" type="application/atom+xml" rel="alternate"
+        <link href="{{ SITEURL }}/{{ CATEGORY_FEED_ATOM|format(slug=category.slug) }}" type="application/atom+xml" rel="alternate"
               title="{{ SITENAME }} {{ category }} ATOM Feed"/>
     {% endif %}