Bump rack from 2.0.6 to 2.0.8 in /examples/sinatra

Bumps [rack](https://github.com/rack/rack) from 2.0.6 to 2.0.8.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.6...2.0.8)

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

examples/sinatra/Gemfile.lock

@@ -8,7 +8,7 @@ GEM
       rack (>= 1.6.2, < 3)
     omniauth-ucam-raven (1.0.1)
       omniauth (~> 1.0)
-    rack (2.0.6)
+    rack (2.0.8)
     rack-protection (2.0.4)
       rack
     sinatra (2.0.4)