[Docs] Fix toctree sorting

[DN6]

What does this PR do?

CI is complaining about a sorting issue in the toctree. It's leading to the quality checks failing on other PRs.
https://github.com/huggingface/diffusers/actions/runs/13506488502/job/37737141072?pr=10859

Just ran make style on it to fix.

Fixes # (issue)

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