ch19-05: Enhance Returning Closures section

[mgacek8]

This PR enhances Returning Closures section from ch19-05.
It mentions the impl Trait syntax for returning closures from functions, as another way except Trait objects.
Previously suggestion to use that was in the compiler error, but the Book depicted only Trait objects as a solution.
Now, two ways of returning a closure from a function are present.

fixes: #1514

Welcome any feedback! (this is my first PR to the Book)

[conradludgate]

I just came to propose this suggestion too. The error log suggests using impl too

$ cargo build
   Compiling functions-example v0.1.0 (file:///projects/functions-example)
error[E0746]: return type cannot have an unboxed trait object
 --> src/lib.rs:1:25
  |
1 | fn returns_closure() -> dyn Fn(i32) -> i32 {
  |                         ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
help: use `impl Fn(i32) -> i32` as the return type, as all return paths are of type `[closure@src/lib.rs:2:5: 2:14]`, which implements `Fn(i32) -> i32`
  |
1 | fn returns_closure() -> impl Fn(i32) -> i32 {
  |                         ^^^^^^^^^^^^^^^^^^^

[chriskrycho]

Thanks for this! I went ahead and did a bit more of a rewrite just now, to tackle the same issue as well as a related one, in #4152, but this PR was nonetheless appreciated!