Change example of ignoring dependencies for yaml

Change constructor injection to setter injection (the same as in examples for xml and php), so it is clear what method call is ignored

Author: Alex Savkov

service_container/optional_dependencies.rst

@@ -76,7 +76,8 @@ call if the service exists and remove the method call if it does not:
         services:
             app.newsletter_manager:
                 class:     AppBundle\Newsletter\NewsletterManager
-                arguments: ['@?app.mailer']
+                calls:
+                    - [setMailer, ['@?app.mailer']]
 
     .. code-block:: xml