alert() not working in codepen, consider alternative onclick functions for tutorial

https://github.com/reactjs/reactjs.org/blob/master/content/tutorial/tutorial.md

At some part of this tutorial (um, line 238 of that md file), I was asked to give the squares an onclick function of alert("click"). However, apparently Chrome has blocked this from running in codepen (A different origin subframe tried to create a JavaScript dialog. This is no longer allowed and was blocked. See https://www.chromestatus.com/feature/5148698084376576 for more details.), and can be confusing to people.

The next best alternative for alert that I can think of is console.log, and logged messages can be seen in both the browser's console and codepen's console.

tutorial/tutorial.md を 238 行目付近で開き、CodePen の例にある正方形の alert("click") ハンドラーを確認してください。console.log のような代替手段がブラウザーのコンソールと CodePen のコンソールの両方で機能することを検証してください。例がブロックされたダイアログをトリガーしなくなり、理解しやすい状態が保たれれば作業は完了です。