specs: simplify the syntax of some factories

spec/factories/service.rb

@@ -1,6 +1,6 @@
 FactoryBot.define do
   factory :service do
-    nom { 'service' }
+    sequence(:nom) { |n| "Service #{n}" }
     organisme { 'organisme' }
     type_organisme { Service.type_organismes.fetch(:association) }
     email { 'email@toto.com' }