views: tell an attestation will be sent only if it is true

Previously the message stating "an attestation will be sent" was
displayed even when the attestation was disabled.

spec/views/instructeur/dossiers/_state_button_motivation.html.haml_spec.rb

@@ -20,6 +20,7 @@ describe 'instructeurs/dossiers/state_button_motivation.html.haml', type: :view
   end
 
   context 'without an attestation preview' do
+    it { expect(rendered).not_to have_text("envoie automatiquement une attestation") }
     it { expect(rendered).not_to have_text("Voir l'attestation") }
   end
 end