Uniformize headers in simple theme

In the simple theme, some templates are using `h1`, others are using
`h2` for the main title of the page (other than the one in the header).
This commit changes that so all of the pages are using `h1`.

pelican/themes/simple/templates/category.html

@@ -3,6 +3,6 @@
 {% block title %}{{ SITENAME }} - {{ category }} category{% endblock %}
 
 {% block content_title %}
-<h2>Articles in the {{ category }} category</h2>
+<h1>Articles in the {{ category }} category</h1>
 {% endblock %}