Fixed broken link in "Handling Authentication Failure"

Author: stacyhorton

cookbook/security/api_key_authentication.rst

@@ -215,7 +215,7 @@ Handling Authentication Failure
 
 In order for your ``ApiKeyAuthentication`` to correctly display a 403
 http status when either bad credentials or authentication fails you will
-need to implement the :class:`Symfony\Component\Security\Http\Authentication\AuthenticationFailureHandlerInterface` on your
+need to implement the :class:`Symfony\\Component\\Security\\Http\\Authentication\\AuthenticationFailureHandlerInterface` on your
 Authenticator. This will provide a method ``onAuthenticationFailure`` which
 you can use to create an error ``Response``.