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.

打开 tutorial/tutorial.md 的第 238 行附近，检查 CodePen 示例中正方形的 alert("click") 处理程序。验证 console.log 之类的替代方案在浏览器控制台和 CodePen 控制台中都能正常工作；当示例不再触发被阻止的对话框且仍然易于理解时，工作就完成了。