Bump rack in /examples/middleman in the bundler group across 1 direct…

…ory (#13)

Bumps the bundler group with 1 update in the /examples/middleman directory: [rack](https://github.com/rack/rack).


Updates `rack` from 2.2.19 to 2.2.20
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.19...v2.2.20)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 2.2.20
  dependency-type: indirect
  dependency-group: bundler
...

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