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.

Apri tutorial/tutorial.md intorno alla riga 238 e controlla l'handler alert("click") dei quadrati nell'esempio CodePen. Verifica che un'alternativa come console.log funzioni sia nella console del browser sia nelle console di CodePen; il lavoro è completato quando l'esempio non attiva più la finestra di dialogo bloccata e rimane comprensibile.