Bump rdoc from 6.6.2 to 6.6.3.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.6.2 to 6.6.3.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.6.2...v6.6.3.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: indirect
...

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

Gemfile.lock

@@ -116,7 +116,7 @@ GEM
     rainbow (3.1.1)
     rake (13.1.0)
     rb-readline (0.5.5)
-    rdoc (6.6.2)
+    rdoc (6.6.3.1)
       psych (>= 4.0.0)
     regexp_parser (2.9.0)
     reline (0.4.3)