Bump rdoc from 6.8.1 to 6.9.0

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.8.1 to 6.9.0.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.8.1...v6.9.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -598,7 +598,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.8.1)
+    rdoc (6.9.0)
       psych (>= 4.0.0)
     recursive-open-struct (1.3.1)
       ostruct