bug #5237 Update authentication.rst (taavit)

This PR was merged into the 2.6 branch.

Discussion
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Update authentication.rst

Added missing ```use``` base class for FoobarEncoder

Commits
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91bdf7e Update authentication.rst
4e0dee6 Update authentication.rst

Author: weaverryan

components/security/authentication.rst

@@ -213,6 +213,7 @@ own, it just needs to follow these rules:
    :method:`Symfony\\Component\\Security\\Core\\Encoder\\BasePasswordEncoder::isPasswordTooLong`
    method for this check::
 
+       use Symfony\Component\Security\Core\Encoder\BasePasswordEncoder;
        use Symfony\Component\Security\Core\Exception\BadCredentialsException;
 
        class FoobarEncoder extends BasePasswordEncoder