doc: fix TokenizersBackend.convert_to_native_format docstring

[lowzhao]

What does this PR do?

Fix docstring spelling mistake TokenizersBackend.convert_to_native_format.

    @classmethod
    def convert_to_native_format(cls, trust_remote_code=False, **kwargs):
        """s          <---- additional s

Likely caused by mistyped ctrl/cmd + s command.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?

Who can review?

Documentation: @stevhliu

[stevhliu]

thanks!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.