dossier: fix upload route of piece_justificative in repetitions

In repetitions, `form.index` doesn't make much sense. Turns out we
don't really need the index of the champ: we can just use the champ id.

app/helpers/champ_helper.rb

@@ -34,7 +34,7 @@ module ChampHelper
 
   def auto_attach_url(form, object)
     if feature_enabled?(:autoupload_dossier_attachments) && object.is_a?(Champ) && object.public?
-      champs_piece_justificative_url(form.index)
+      champs_piece_justificative_url(object.id)
     end
   end
 end