Adds toString explanation

I noticed that there is a toString() function at https://github.com/dart-lang/one-hour-codelab/blob/master/web/4-classbadge/piratebadge.dart#L57, but isn't mentioned in the tutorial, so I added a brief explanation.
Hi @kwalrath, thank you for the recommendations! This reads very well now. I went ahead and made the changes.