minor symfony#7910 Update optional_dependencies.rst (javihgil)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes symfony#7910).

Discussion
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Update optional_dependencies.rst

Update injected service name according to the previous service definition "app.mailer" instead of "my_mailer" in XML and PHP blocks.

Commits
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eb36653 Update optional_dependencies.rst

Author: xabbuh

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
                 ),
             ))