[Docs] Update Imagen Video paper link in schedulers

[delmalih]

What does this PR do?

This PR updates the link to the Imagen Video paper in the docstrings of multiple schedulers. The previous link (https://imagen.research.google/video/paper.pdf) was replaced with the Hugging Face papers link (https://huggingface.co/papers/2210.02303) to ensure accessibility.

Testing

• Code passes make style
• Code passes make quality
• Code passes make fix-copies
• Existing tests still pass

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?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? issue.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? (N/A - documentation only changes)

Who can review?

cc @stevhliu for review