chore(deps): bump tornado from 5.0.1 to 6.3.3 in /docs (#1266)

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.0.1 to 6.3.3.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.0.1...v6.3.3)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
...

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

docs/docs-requirements.txt

@@ -19,5 +19,5 @@ requests==2.18.4
 selenium==3.4.3
 six==1.10.0
 SQLAlchemy==1.1.13
-tornado==5.0.1
+tornado==6.3.3