fix(peft): correct typo in flattening operation error messages

[leaderofARS]

What does this PR do?

This PR corrects a minor spelling typo in the PEFT integration error messages. Specifically, it corrects the misspelling of "flattening operation" (previously written as "flatteing operatio") in two NotImplementedError messages within src/transformers/integrations/peft.py to improve clarity.

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• I confirm that this is not a pure code agent PR.

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?
• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@BenjaminBossan
@githubnemo

[github-actions]

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=46538&sha=43379b

[leaderofARS]

The tests failed due to a transient network error (httpx.RemoteProtocolError). Could someone re-run the CI jobs?

[Rocketknight1]

Sorry, we're trying to avoid these small typo PRs right now! We'll do an agent pass at some point to just fix all of them, and in the meantime we want to keep PRs for significant stuff because our maintainers are struggling with the flood of agent submissions