Bump tornado in /docs in the pip group across 1 directory (#406)

Bumps the pip group with 1 update in the /docs directory: [tornado](https://github.com/tornadoweb/tornado).


Updates `tornado` from 6.4.1 to 6.4.2
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.1...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: indirect
  dependency-group: pip
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>