Update optional_dependencies.rst

Author: javihgil

service_container/optional_dependencies.rst

@@ -91,7 +91,7 @@ call if the service exists and remove the method call if it does not:
 
                 <service id="app.newsletter_manager" class="AppBundle\Newsletter\NewsletterManager">
                     <call method="setMailer">
-                        <argument type="service" id="my_mailer" on-invalid="ignore"/>
+                        <argument type="service" id="app.mailer" on-invalid="ignore"/>
                     </call>
                 </service>
             </services>
@@ -110,7 +110,7 @@ call if the service exists and remove the method call if it does not:
             ->register('app.newsletter_manager', NewsletterManager::class)
             ->addMethodCall('setMailer', array(
                 new Reference(
-                    'my_mailer',
+                    'app.mailer',
                     ContainerInterface::IGNORE_ON_INVALID_REFERENCE
                 ),
             ))