[Security][Authentication] Fix instructions for creating password encoders

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Please correct me if I am wrong, but it seems that the code has changed after https://github.com/symfony/symfony-docs/issues/3003. There is no `BasePasswordEncoder::checkPasswordLength()` method. Same seems to apply to 2.4. Maybe the implementation was changed to make it bc?