Fix ui test for nll

src/test/ui/kindck/kindck-impl-type-params.nll.stderr

@@ -91,6 +91,10 @@ note: required because of the requirements on the impl of `Gettable<Foo>` for `S
 LL | impl<T: Send + Copy + 'static> Gettable<T> for S<T> {}
    |                                ^^^^^^^^^^^     ^^^^
    = note: required for the cast to the object type `dyn Gettable<Foo>`
+help: consider annotating `Foo` with `#[derive(Copy)]`
+   |
+LL |     #[derive(Copy)]
+   |
 
 error: aborting due to 6 previous errors