Fix/misc docs and tests (#19652)

## Summary
This PR makes a few small consistency fixes:

- Fix outdated GitHub links that still point to the old
`tailwindcss/tailwindcss` org and update them to
`tailwindlabs/tailwindcss` (README + contributing docs + PR template).
- Fix punctuation in the contributing guide (“i.e., …”).
- Update two test titles to use “cannot” instead of “can not” for
consistency.

No behavior changes.

## Test plan
- Link check (manual): clicked the updated GitHub links in `README.md`,
`.github/CONTRIBUTING.md`, and `.github/PULL_REQUEST_TEMPLATE.md` to
confirm they resolve correctly.
- (Optional) `pnpm test` — not required since changes are
docs/test-title-only.

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Co-authored-by: Robin Malfait <malfait.robin@gmail.com>