minor #13598 Fix PHP example on ExpressionLanguage Injection (jderusse)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Fix PHP example on ExpressionLanguage Injection

Current example does not work. generate `service(App\Service)` instead of `service('App\\Service')`
Missing quotes and `\` escape

Commits
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394385d Fix PHP example on ExpressionLanguage Injection

Author: javiereguiluz

service_container/expression_language.rst

@@ -67,7 +67,7 @@ to another service: ``App\Mailer``. One way to do this is with an expression:
             $services->set(MailerConfiguration::class);
 
             $services->set(Mailer::class)
-                ->args([expr(sprintf('service(%s).getMailerMethod()', MailerConfiguration::class))]);
+                ->args([expr(sprintf('service(%s).getMailerMethod()', var_export(MailerConfiguration::class, true)))]);
         };