Bump rdoc from 6.4.0 to 6.4.1.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.4.0 to 6.4.1.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.4.0...v6.4.1.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: direct:development
...

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

Gemfile.lock

@@ -15,11 +15,11 @@ GEM
     parser (3.1.2.0)
       ast (~> 2.4.1)
     power_assert (2.0.1)
-    psych (4.0.4)
+    psych (5.1.2)
       stringio
     rainbow (3.1.1)
     rake (13.0.6)
-    rdoc (6.4.0)
+    rdoc (6.4.1.1)
       psych (>= 4.0.0)
     regexp_parser (2.5.0)
     rexml (3.2.5)
@@ -62,7 +62,7 @@ GEM
     simplecov-html (0.12.3)
     simplecov-lcov (0.8.0)
     simplecov_json_formatter (0.1.4)
-    stringio (3.0.2)
+    stringio (3.0.6)
     test-unit (3.5.3)
       power_assert
     unicode-display_width (2.1.0)