Update peft_finetuning.ipynb

[ghLcd9dG]

What does this PR do?

This PR solve an old quesiton.

prepare_model_for_int8_training has been deprecated for quite some time, with PEFT v0.10.0, it has been removed. Please use prepare_model_for_kbit_training instead. @BenjaminBossan

Detail can be seen here. https://github.com/huggingface/peft/issues/1583

Fixes # (issue)

Feature/Issue validation/testing

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• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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