Docs: correct default value of 'use_llmlingua2'

[kexplo]

What does this PR do?

The default argument for the 'use_llmlingua2' parameter is False, but the docstring is incorrectly written as True. The DOCUMENT.md file is correctly written as False.

https://github.com/microsoft/LLMLingua/blob/40ac969a82f162b3eb0b8e1f1416756d442e4eec/DOCUMENT.md?plain=1#L283

Before submitting

• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes?
  • the document already describes it's default value is False:
    • https://github.com/microsoft/LLMLingua/blob/40ac969a82f162b3eb0b8e1f1416756d442e4eec/DOCUMENT.md?plain=1#L283
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

@iofu728 @mydmdm

[iofu728]

Thanks for your help.