use section instead of categories for breadcrumb

froide_govplan/templates/froide_govplan/detail.html

@@ -10,7 +10,7 @@
 
 {% block govplan_breadcrumbs %}
 <li class="breadcrumb-item">
-  <a href="{{ object.categories.first.get_absolute_url }}">{{ object.categories.first.name }}</a>
+  <a href="{{ section.get_absolute_url }}">{{ section.title}}</a>
 </li>
 <li class="breadcrumb-item active" aria-current="page">
   <a href="{{ object.get_absolute_url }}">{{ object.title }}</a>