Rollup merge of #40144 - MajorBreakfast:patch-7, r=frewsxcv

Unit-like structs doc: Improve code sample

r? @steveklabnik

BTW it seems that
```Rust
let p = Proton {};
```
compiles without an error. That's why I didn't add it to the example. It's about consistency anyway.

diff --cc src/doc/book/src/structs.md
Simple merge