build: bump concurrent-ruby from 1.1.5 to 1.1.6 in /docs

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.1.5 to 1.1.6.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ruby-concurrency/concurrent-ruby/compare/v1.1.5...v1.1.6

)

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