tour: new concept in code example is unnamed

[Samuelodan]

Context: https://go.dev/tour/moretypes/9

I noticed that anonymous structs are silently introduced in this lesson. It made me think I missed it from an earlier lesson, but apparently, I didn't. It was, in fact, new.

So, can we add a note in the lesson, at least, mentioning that anonymous structs are used?