minor #6905 Change example of ignoring dependencies for yaml (Integrity-178B)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6905).

Discussion
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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

Commits
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282d0fc Change example of ignoring dependencies for yaml

Author: xabbuh

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