Auto merge of #147143 - estebank:verbose-ret-type, r=fee1-dead

Make replacement suggestion `_` in type verbose

```
error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
  --> $DIR/in-signature.rs:6:21
   |
LL | fn arr_fn() -> [u8; _] {
   |                     ^ not allowed in type signatures
   |
help: replace with the correct return type
   |
LL - fn arr_fn() -> [u8; _] {
LL + fn arr_fn() -> [u8; 3] {
   |
```

Author: bors