build(deps-dev): update rdoc requirement from = 6.4.0 to 6.5.0

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.4.0...v6.5.0)

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

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

Gemfile

@@ -15,7 +15,7 @@ group :development do
 
   # documentation
   gem "hoe-markdown", "= 1.4.0"
-  gem "rdoc", "= 6.4.0"
+  gem "rdoc", "6.5.0"
   gem "psych", "~> 4.0" # psych 5 isn't building in places yet https://github.com/ruby/setup-ruby/issues/409
 
   # parser generator