Update chess_game.rb

[Horlamedhey]

The method 'nick_name' is not consistent with the instructions' version (nickname), and since 'nickname' is a whole word, it makes sense without the underscore.

Discussion for approval: https://forum.exercism.org/t/update-chess-game-rb/14696

[github-actions]

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[kotp]

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[kotp]

Superceded by #1769.