docs(pre-commit): bump pre-commit hook revision to v0.21.0

[Bubu-Droid]

Bumped the version of pre-commit hook revision in pre-commit.md.
I'm sorry, I have OCD. I've ventured on a journey of opening PRs to every prek hook
I use whose revision is not updated in their docs, to get a good sleep tonight.

[google-cla]

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[Bubu-Droid]

Perhaps we should add a workflow script which would update the version in pre-commit docs using regex matching.