Fix instance variable used for admin set form

This doesn’t have any behavioural change, but the instance variable used
for remembering the admin set form was mistakenly named
`@administrative_set_model` (compare `@admin_set_model`) and not
`@administrative_set_form`.

Author: marrus-sh

lib/hyrax/configuration.rb

@@ -894,7 +894,7 @@ def index_field_mapper
     ##
     # @return [Class]
     def administrative_set_form
-      @administrative_set_model ||= Hyrax::Forms::AdministrativeSetForm
+      @administrative_set_form ||= Hyrax::Forms::AdministrativeSetForm
     end
 
     attr_writer :file_set_form