Typo in documentation.
Under parameters for `GenerationMixin.generate()`
there is no new bullet point for `prefix_allowed_tokens_fn`, and the content for it is under [`diversity_penalty`](https://huggingface.co/docs/transformers/v4.13.0/en/main_classes/model#transformers.generation_utils.GenerationMixin.generate.diversity_penalty).

This typo only exists for versions after the document UI update(>= 4.13.0).
@sgugger 
Thanks for flagging, the PR mentioned above will fix this issue.