Fix reference to translation of "Subgroups"

app/views/activities/show.html.haml

         - if @assignable_subgroups.any?
           %tr
             %td
-              = t 'activerecord.attributes.activity.subgroups'
+              = t 'activerecord.models.subgroup.other'
 
             %td
               = @assignable_subgroups.join(', ')
   - if @activity.subgroups.any?
     .panel.panel-default
       .panel-heading
-        = t 'activerecord.attrs.activities.subgroups'
+        = t 'activerecord.models.subgroup.other'
       .panel-body
         = select_tag(:subgroup_filter, options_for_select(@subgroup_ids), class: 'form-control subgroup-filter')
 

config/locales/aardbei_en.yml

         one: "Member"
         other: "Members"
 
+      subgroup:
+        one: "Subgroup"
+        other: "Subgroups"
+
     attributes:
       person:
         first_name: "First name"