[#3897] Fix error message

Someone having the role called "Project admin" may not have the
permission to archive subprojects. Having an error message asking to
contact the project administrator will leave them puzzled as they are
the project administrator.

The real person able to help here is the project administrator. Reflect
this in the error message.

config/locales/en.yml

@@ -768,7 +768,7 @@ en:
           foreign_wps_reference_version: 'Work packages in non descendant projects reference versions of the project or its descendants.'
           attributes:
             base:
-              archive_permission_missing_on_subprojects: "You do not have the permissions required to archive all sub-projects. Please contact your project administrator."
+              archive_permission_missing_on_subprojects: "You do not have the permissions required to archive all sub-projects. Please contact an administrator."
             types:
               in_use_by_work_packages: "still in use by work packages: %{types}"
             enabled_modules: