themes: Fix format issue in simple template

* The `simple` template that is bundled with Pelican has been shown to
  suffer from the problem described in getpelican#2489. This has also been
  reported as a bug in the Debian Bug tracer:
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=919636

* This commit fixes the problem by updating the `simple` template to the
  new way of interpolating variables via the Jinja2 `format` helper
  function.

Signed-off-by: mr.Shu <mr@shu.io>

pelican/themes/simple/templates/base.html

@@ -17,16 +17,16 @@
         <link href="{{ FEED_DOMAIN }}/{% if FEED_RSS_URL %}{{ FEED_RSS_URL }}{% else %}{{ FEED_RSS }}{% endif %}" type="application/rss+xml" rel="alternate" title="{{ SITENAME }} RSS Feed" />
         {% endif %}
         {% if CATEGORY_FEED_ATOM and category %}
-        <link href="{{ FEED_DOMAIN }}/{% if CATEGORY_FEED_ATOM_URL %}{{ CATEGORY_FEED_ATOM_URL|format(category.slug) }}{% else %}{{ CATEGORY_FEED_ATOM|format(category.slug) }}{% endif %}" type="application/atom+xml" rel="alternate" title="{{ SITENAME }} Categories Atom Feed" />
+        <link href="{{ FEED_DOMAIN }}/{% if CATEGORY_FEED_ATOM_URL %}{{ CATEGORY_FEED_ATOM_URL|format(slug=category.slug) }}{% else %}{{ CATEGORY_FEED_ATOM|format(slug=category.slug) }}{% endif %}" type="application/atom+xml" rel="alternate" title="{{ SITENAME }} Categories Atom Feed" />
         {% endif %}
         {% if CATEGORY_FEED_RSS and category %}
-        <link href="{{ FEED_DOMAIN }}/{% if CATEGORY_FEED_RSS_URL %}{{ CATEGORY_FEED_RSS_URL|format(category.slug) }}{% else %}{{ CATEGORY_FEED_RSS|format(category.slug) }}{% endif %}" type="application/rss+xml" rel="alternate" title="{{ SITENAME }} Categories RSS Feed" />
+        <link href="{{ FEED_DOMAIN }}/{% if CATEGORY_FEED_RSS_URL %}{{ CATEGORY_FEED_RSS_URL|format(slug=category.slug) }}{% else %}{{ CATEGORY_FEED_RSS|format(slug=category.slug) }}{% endif %}" type="application/rss+xml" rel="alternate" title="{{ SITENAME }} Categories RSS Feed" />
         {% endif %}
         {% if TAG_FEED_ATOM and tag %}
-        <link href="{{ FEED_DOMAIN }}/{% if TAG_FEED_ATOM_URL %}{{ TAG_FEED_ATOM_URL|format(tag.slug) }}{% else %}{{ TAG_FEED_ATOM|format(tag.slug) }}{% endif %}" type="application/atom+xml" rel="alternate" title="{{ SITENAME }} Tags Atom Feed" />
+        <link href="{{ FEED_DOMAIN }}/{% if TAG_FEED_ATOM_URL %}{{ TAG_FEED_ATOM_URL|format(slug=tag.slug) }}{% else %}{{ TAG_FEED_ATOM|format(slug=tag.slug) }}{% endif %}" type="application/atom+xml" rel="alternate" title="{{ SITENAME }} Tags Atom Feed" />
         {% endif %}
         {% if TAG_FEED_RSS and tag %}
-        <link href="{{ FEED_DOMAIN }}/{% if TAG_FEED_RSS_URL %}{{ TAG_FEED_RSS_URL|format(tag.slug) }}{% else %}{{ TAG_FEED_RSS|format(tag.slug) }}{% endif %}" type="application/rss+xml" rel="alternate" title="{{ SITENAME }} Tags RSS Feed" />
+        <link href="{{ FEED_DOMAIN }}/{% if TAG_FEED_RSS_URL %}{{ TAG_FEED_RSS_URL|format(slug=tag.slug) }}{% else %}{{ TAG_FEED_RSS|format(slug=tag.slug) }}{% endif %}" type="application/rss+xml" rel="alternate" title="{{ SITENAME }} Tags RSS Feed" />
         {% endif %}
         {% endblock head %}
 </head>