docs: update ACL Anthology URL in CITATION.cff

[irfaan101]

What does this PR do?

This PR updates the outdated ACL Anthology URL in the CITATION.cff file. The repository currently points to the legacy link format https://www.aclweb.org/anthology/2020.emnlp-demos.6. This change updates it to the current, official permanent canonical domain format: https://aclanthology.org/2020.emnlp-demos.6/.

Fixes # (Direct citation metadata enhancement - no tracking issue)

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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?

Tagging the documentation maintainer for a quick metadata review:

@stevhliu

[stevhliu]

👍