minor #10120 Update custom_password_authenticator.rst (rpodwika)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes #10120).

Discussion
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Update custom_password_authenticator.rst

In the example there's no encoder factory defined, only encoder itself.

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Commits
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f096458 Update custom_password_authenticator.rst

Author: javiereguiluz

security/custom_password_authenticator.rst

@@ -69,7 +69,7 @@ the user::
                 if ('' === ($givenPassword = $token->getCredentials())) {
                     throw new BadCredentialsException('The given password cannot be empty.');
                 }
-                if (!$this->encoderFactory->getEncoder($user)->isPasswordValid($user->getPassword(), $givenPassword, $user->getSalt())) {
+                if (!$this->encoder->isPasswordValid($user->getPassword(), $givenPassword, $user->getSalt())) {
                     throw new BadCredentialsException('The given password is invalid.');
                 }
             }