Bump rdoc from 6.13.0 to 6.13.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.13.0 to 6.13.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.13.0...v6.13.1)

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

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

Author: dependabot[bot]

Diff for: Gemfile.lock

@@ -147,7 +147,7 @@ GEM
       rack (>= 1.3)
     rainbow (3.1.1)
     rake (13.2.1)
-    rdoc (6.13.0)
+    rdoc (6.13.1)
       psych (>= 4.0.0)
     regexp_parser (2.10.0)
     reline (0.6.0)