Fix the agenda backcover representation

This commit makes the "backcover" elements to appear as simple bold text instead
of H2 headers.
In this way PDF readers that show a selectable index, will not show back covers
as chapters (because they're not).

Author: rascasoft

templates/agenda.md.j2

@@ -17,7 +17,7 @@ background-image: "{{ pandoc_agenda_background_image }}"
 {% else %}
 {% if slide.backcover is defined and slide.backcover | bool and slide.subtitle is defined %}
 
-## {{ slide.subtitle }}
+**{{ slide.subtitle }}**
 {% else %}
 
 - {{ slide.title }}