minor symfony#11982 Fixed Swiftmailer PHP configuration (dborsatto)

This PR was submitted for the 4.0 branch but it was merged into the 4.2 branch instead (closes symfony#11982).

Discussion
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Fixed Swiftmailer PHP configuration

Just a fix for a PHP snippet in the Swiftmailer reference configuration. I chose 4.0 as target as it's the branch when this snippet was introduced, let me know if I need to change anything.

Commits
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e1ad33e Fixed Swiftmailer PHP configuration

Author: javiereguiluz

reference/configuration/swiftmailer.rst

@@ -388,7 +388,7 @@ alternatives based on the :ref:`service binding <services-binding>` feature:
             ->setPublic(true)
             ->setBindings([
                 // this injects the second mailer when this service type-hints constructor arguments with \Swift_Mailer
-                \Swift_Mailer => '@swiftmailer.mailer.second_mailer',
+                \Swift_Mailer::class => '@swiftmailer.mailer.second_mailer',
                 // this injects the second mailer when this service has a constructor argument called $specialMailer
                 '$specialMailer' => '@swiftmailer.mailer.second_mailer',
             ])