* Refactor dir.rb sample
The original (1998) sample with a for-loop and use of case/when isn't what we'd write nowadays
* [DOC] Update sample/dir.rb [ci skip]
Do not leave a `Dir` opened.
* [DOC] Update sample/dir.rb [ci skip]
Fix ArgumentError.
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Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>