bug #5220 Fix example namespace (lepiaf)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix example namespace

Hi,

I've found a mistake on example in security component.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.0 to 3.0
| Fixed tickets | none

Commits
-------

83e2616 Fix example namespace

components/security/authentication.rst

@@ -71,7 +71,7 @@ The default authentication manager is an instance of
 
     use Symfony\Component\Security\Core\Authentication\AuthenticationProviderManager;
 
-    // instances of Symfony\Component\Security\Core\Authentication\AuthenticationProviderInterface
+    // instances of Symfony\Component\Security\Core\Authentication\Provider\AuthenticationProviderInterface
     $providers = array(...);
 
     $authenticationManager = new AuthenticationProviderManager($providers);