Bump rdoc from 6.5.1.1 to 6.7.0 (#389)

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.5.1.1 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@v6.5.1.1...v6.7.0)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -160,7 +160,7 @@ GEM
       forwardable-extended (~> 2.6)
     power_assert (2.0.4)
     profile (0.4.0)
-    psych (5.1.2)
+    psych (5.2.0)
       stringio
     public_suffix (5.1.1)
     racc (1.8.1)
@@ -177,7 +177,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.5.1.1)
+    rdoc (6.7.0)
       psych (>= 4.0.0)
     recursive-open-struct (1.3.1)
       ostruct
@@ -233,7 +233,7 @@ GEM
     simplecov-html (0.12.3)
     simplecov_json_formatter (0.1.4)
     slack-notifier (2.4.0)
-    stringio (3.1.1)
+    stringio (3.1.2)
     swd (2.0.2)
       activesupport (>= 3)
       attr_required (>= 0.0.5)