minor #15286 [Security] Fixes typo in experimental authenticators (jmleroux)

This PR was submitted for the 5.x branch but it was merged into the 5.2 branch instead.

Discussion
----------

[Security] Fixes typo in experimental authenticators

`$email` was not defined in the example.

Commits
-------

0df9393 [Security] Fixes typo in experimental authenticators

Author: OskarStark

security/experimental_authenticators.rst

@@ -582,7 +582,7 @@ would initialize the passport like this::
             // ... validate no parameter is empty
 
             return new Passport(
-                new UserBadge($email),
+                new UserBadge($username),
                 new PasswordCredentials($password),
                 [new CsrfTokenBadge('login', $csrfToken)]
             );