Reflect by-value object safety in the unstable book.

Author: qnighy

src/doc/unstable-book/src/language-features/unsized-locals.md

@@ -101,9 +101,9 @@ fn main() {
 }
 ```
 
-And `Foo` will also be object-safe. However, this object-safety is not yet implemented.
+And `Foo` will also be object-safe.
 
-```rust,ignore
+```rust
 #![feature(unsized_locals)]
 
 trait Foo {
@@ -119,8 +119,6 @@ fn main () {
 }
 ```
 
-Unfortunately, this is not implemented yet.
-
 One of the objectives of this feature is to allow `Box<dyn FnOnce>`, instead of `Box<dyn FnBox>` in the future. See [#28796] for details.
 
 [#28796]: https://github.com/rust-lang/rust/issues/28796