Bump rack from 2.0.6 to 2.2.3 in /examples/rails_app

Bumps [rack](https://github.com/rack/rack) from 2.0.6 to 2.2.3.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.6...2.2.3)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

examples/rails_app/Gemfile.lock

@@ -106,7 +106,7 @@ GEM
       mini_portile2 (~> 2.4.0)
     public_suffix (3.0.3)
     puma (3.12.1)
-    rack (2.0.6)
+    rack (2.2.3)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.2.1)