Bump rdoc from 6.6.1 to 6.6.3.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.6.1 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@v6.6.1...v6.6.3.1)

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

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

Gemfile.lock

@@ -63,7 +63,7 @@ GEM
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     power_assert (2.0.3)
-    psych (5.1.1.1)
+    psych (5.1.2)
       stringio
     public_suffix (5.0.4)
     racc (1.7.3)
@@ -72,7 +72,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rdoc (6.6.1)
+    rdoc (6.6.3.1)
       psych (>= 4.0.0)
     regexp_parser (2.8.3)
     reline (0.4.1)