docs: update README instructions for updating WPT (#32124)

I noticed in #32123 that there are two issues with the docs for updating
Web Platform Tests:

- `./mach update-wpt --sync` fails with an error (`Are you sure you
don't want a patch?`) because it expects a `--patch` arg.
- The `tests/wpt/meta-legacy-layout/` test results need to be updated
with a separate command.

This updates `tests/wpt/README.md` with more complete instructions.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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