Fix PHP example on ExpressionLanguage Injection

[jderusse]

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

[javiereguiluz]

Jérémy, thanks for spotting this error. While merging I replaced the fix by:

            $services->set(Mailer::class)
                ->args([expr("service('App\\Mail\\MailerConfiguration').getMailerMethod()")]);

Although your solution is nice ... it introduces added complexity to an already complex example. So, I prefer to use the same syntax as in YAML and XML. It's true that we lost the nice ::class constant ... but the example know shows more clearly what you need to configure. It's up to the reader to come up with advanced alternatives like the one you showed. Thanks!