Bump tornado from 6.4 to 6.4.1 in /doc

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.0...v6.4.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

doc/requirements.txt

@@ -254,7 +254,7 @@ tomli==2.0.1
     #   build
     #   pip-tools
     #   pyproject-hooks
-tornado==6.4
+tornado==6.4.1
     # via
     #   ipykernel
     #   jupyter-client