chore: update gem: rdoc
chore: update gem: rdoc

- Update gem: `rdoc`
  - From version: `4.1.1 - January 09, 2014 (717 KB)`
  - To version: `6.2.1 - December 23, 2019 (551 KB)`

More details:

- https://rubygems.org/gems/rdoc

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