add example invocation of fmt! for struct in chapter 8

Chapter 8 mentions using `fmt!` to print a struct, and gives example output, but not an example invocation. `fmt!` was already introduced in ch 5, but there's a lot of info packed into 6 and 7, so I think it's probably a good idea to reestablish it.

side note: my understanding from the 0.8 release note is that the new `format!` will eventually replace `fmt!`; do you think these should be changed to use it instead?