build(deps): bump concurrent-ruby from 1.1.8 to 1.1.9 in /docs

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.1.8 to 1.1.9.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](ruby-concurrency/concurrent-ruby@v1.1.8...v1.1.9)

---
updated-dependencies:
- dependency-name: concurrent-ruby
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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

docs/Gemfile.lock

@@ -16,7 +16,7 @@ GEM
     colorator (1.1.0)
     commonmarker (0.17.13)
       ruby-enum (~> 0.5)
-    concurrent-ruby (1.1.8)
+    concurrent-ruby (1.1.9)
     dnsruby (1.61.5)
       simpleidn (~> 0.1)
     em-websocket (0.5.2)