Add missing ul tag in tags list in simple theme

There are some li elements generated without a ul/ol around them.
Besides being malformed html, this results in there being no margin that
lists normally have.

pelican/themes/simple/templates/tags.html

@@ -4,7 +4,9 @@
 
 {% block content %}
     <h1>Tags for {{ SITENAME }}</h1>
+    <ul>
     {%- for tag, articles in tags|sort %}
         <li><a href="{{ SITEURL }}/{{ tag.url }}">{{ tag }}</a> ({{ articles|count }})</li>
     {% endfor %}
+    </ul>
 {% endblock %}