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.

打开教程并找到 calculateWinner helper，然后检查当前遍历 iterable 对象的 for 语句。将该遍历更新为使用 ES6 for...of 语法，并验证教程的行为保持不变。