[Docs] fix docstring typo in SD3 pipeline

[shenzhiy21]

What does this PR do?

Fixes #9761

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

Thanks for fixing!

[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.

[a-r-r-o-w]

@shenzhiy21 Could you run make style and push the changes to fix the quality tests? Please make sure you're using ruff version 0.1.5

[shenzhiy21]

Completed! Thank you.