Update chess_game.rb

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.

exercises/concept/chess-game/chess_game.rb

@@ -6,7 +6,7 @@ def self.valid_square?(rank, file)
     raise "Please implement the Chess.valid_square? method"
   end
 
-  def self.nick_name(first_name, last_name)
+  def self.nickname(first_name, last_name)
     raise "Please implement the Chess.nick_name method"
   end