fix(code): fix on constructor argument

Co-Authored-By: Guikingone <guillaume.loulier@hotmail.fr>

Author: zanbaldwin

service_container/injection_types.rst

@@ -261,7 +261,7 @@ this approach allow you to make a service immutable::
     {
         private $mailer;
         
-        public function __construct(MailerInterface $mailer)
+        public function __construct(?MailerInterface $mailer = null)
         {
             $this->mailer = $mailer;
         }