Update Tutorial to use ES6 syntax to iterate over lines

The tutorial uses a for statement to iterate through an iterable Object in the calculateWinner helper function. This can be replaced with an es6 for ... of. The tangible benefit of using for ... of over for when you're working with an iterable object is that for ... of decreases the incidence of off-by-one errors.

I've prepared a fork for what this would look like here: https://github.com/reactjs/reactjs.org/compare/master...arg4:patch-2.

Thanks.

開啟教學並找到 calculateWinner helper，接著檢查目前遍歷 iterable 物件的 for 陳述式。將該迭代更新為使用 ES6 for...of 語法，並確認教學的行為維持不變。