Bump rdoc from 6.6.3.1 to 6.7.0

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.6.3.1 to 6.7.0.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.6.3.1...v6.7.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -82,7 +82,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.6.3.1)
+    rdoc (6.7.0)
       psych (>= 4.0.0)
     regexp_parser (2.9.2)
     rexml (3.2.9)