Bump rack from 2.2.6.4 to 2.2.8.1 in /examples/swagger_demo/doc (#108)

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
...

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examples/swagger_demo/doc/Gemfile.lock

@@ -89,7 +89,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.10.0)
     public_suffix (4.0.6)
-    rack (2.2.6.4)
+    rack (2.2.8.1)
     rb-fsevent (0.9.8)
     rb-inotify (0.9.8)
       ffi (>= 0.5.0)