minor #7964 Update guard_authentication.rst (javihgil)

This PR was submitted for the 3.2 branch but it was merged into the 2.8 branch instead (closes #7964).

Discussion
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Update guard_authentication.rst

"Or, extend the simpler :class:`Symfony\\Component\\Security\\Guard\\AbstractGuardAuthenticator`. This requires you to implement seven methods::" not six.

GuardAuthenticatorInterface defines 8 methods, and AbstractGuardAuthenticator implements only the createAuthenticatedToken method:

http://api.symfony.com/3.2/Symfony/Component/Security/Guard/AbstractGuardAuthenticator.html
http://api.symfony.com/3.2/Symfony/Component/Security/Guard/AbstractGuardAuthenticator.html

Thanks!

Commits
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2fa5c7e Update guard_authentication.rst

Author: xabbuh

security/guard_authentication.rst

@@ -148,7 +148,7 @@ user (if any).
 To create a custom authentication system, just create a class and make it implement
 :class:`Symfony\\Component\\Security\\Guard\\GuardAuthenticatorInterface`. Or, extend
 the simpler :class:`Symfony\\Component\\Security\\Guard\\AbstractGuardAuthenticator`.
-This requires you to implement six methods::
+This requires you to implement seven methods::
 
     // src/AppBundle/Security/TokenAuthenticator.php
     namespace AppBundle\Security;