Bump rack from 2.2.6.4 to 2.2.8.1 in /docs

Bumps [rack](https://github.com/rack/rack) from 2.2.6.4 to 2.2.8.1.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.4...v2.2.8.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

docs/Gemfile.lock

@@ -91,7 +91,7 @@ GEM
     parallel (1.19.2)
     public_suffix (4.0.6)
     racc (1.6.2)
-    rack (2.2.6.4)
+    rack (2.2.8.1)
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)