minor #20261 [DependencyInjection] Fix code example (MrYamous)

This PR was submitted for the 7.1 branch but it was merged into the 6.4 branch instead.

Discussion
----------

[DependencyInjection] Fix code example

Fix code example, property double declaration with constructor property promotion

Commits
-------

bfaa54a service-container/fix-code-example

Author: javiereguiluz

service_container.rst

@@ -407,12 +407,11 @@ example, suppose you want to make the admin email configurable:
       class SiteUpdateManager
       {
           // ...
-    +    private string $adminEmail;
 
           public function __construct(
               private MessageGenerator $messageGenerator,
               private MailerInterface $mailer,
-    +        private string $adminEmail
+    +         private string $adminEmail
           ) {
           }