Bump rdoc from 6.3.2 to 6.4.0 (#2254)

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.3.2 to 6.4.0.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](https://github.com/ruby/rdoc/compare/v6.3.2...v6.4.0)

---
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

@@ -124,6 +124,8 @@ GEM
     pry-byebug (3.9.0)
       byebug (~> 11.0)
       pry (~> 0.13.0)
+    psych (4.0.3)
+      stringio
     public_suffix (4.0.6)
     qr4r (0.6.1)
       mojo_magick (~> 0.6.5)
@@ -136,7 +138,8 @@ GEM
       rack (>= 1.0, < 3)
     rainbow (3.0.0)
     rake (13.0.6)
-    rdoc (6.3.2)
+    rdoc (6.4.0)
+      psych (>= 4.0.0)
     regexp_parser (2.1.1)
     reline (0.2.7)
       io-console (~> 0.5)
@@ -189,6 +192,7 @@ GEM
       tilt (~> 2.0)
     slack-notifier (2.4.0)
     sqlite3 (1.4.2)
+    stringio (3.0.1)
     sync (0.5.0)
     term-ansicolor (1.7.1)
       tins (~> 1.0)