minor #1558 Update guard_authentication.rst (symac)

This PR was merged into the 2.2.x-dev branch.

Discussion
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Update guard_authentication.rst

I am still working on it to get authentication working in my project so my suggestion might be false, but I believe a ```use``` is missing in the header of this example.

Commits
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50da4b33 Update guard_authentication.rst

doc/cookbook/guard_authentication.rst

@@ -34,6 +34,7 @@ AbstractGuardAuthenticator. This requires you to implement six methods:
     use Symfony\Component\Security\Guard\AbstractGuardAuthenticator;
     use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
     use Symfony\Component\Security\Core\Exception\AuthenticationException;
+    use Symfony\Component\Security\Core\Encoder\EncoderFactoryInterface;
 
     class TokenAuthenticator extends AbstractGuardAuthenticator
     {