minor #15766 Adds proper escaping for php service config format (faizanakram99)

This PR was submitted for the 5.4 branch but it was merged into the 4.4 branch instead.

Discussion
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Adds proper escaping for php service config format

php configs need `\\\\`, `\\` are not enough.

`"service('App\\Mailer')"` compiles into `AppMailer` ,  `"service('App\\\\Mailer')"` compiles correctly into `App\Mailer`

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Commits
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88c25e1 Adds proper escaping for php service config format

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("service('App\\Mail\\MailerConfiguration').getMailerMethod()")]);
+                ->args([expr("service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()")]);
         };
 
 To learn more about the expression language syntax, see :doc:`/components/expression_language/syntax`.