Bump rack from 2.2.6.3 to 2.2.6.4 in /docs (#4147)

Bumps [rack](https://github.com/rack/rack) from 2.2.6.3 to 2.2.6.4.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.3...v2.2.6.4)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: yuenmichelle1 <michelle@zooniverse.org>

docs/Gemfile.lock

@@ -94,7 +94,7 @@ GEM
  parslet (2.0.0)
  public_suffix (4.0.7)
  racc (1.6.1)
- rack (2.2.6.3)
+ rack (2.2.6.4)
  rb-fsevent (0.11.1)
  rb-inotify (0.10.1)
  ffi (~> 1.0)