Bump rack from 2.0.6 to 2.0.8 in /docs (#112)

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] <[email protected]>

Author: dependabot[bot]

docs/Gemfile.lock

@@ -100,7 +100,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.19.0)
     public_suffix (4.0.1)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-livereload (0.3.17)
       rack
     rb-fsevent (0.10.3)