Rollup merge of rust-lang#130370 - kpreid:patch-2, r=compiler-errors

unstable-book: `trait_upcasting` example should not have `#![allow(incomplete_features)]`

Tracking issue: rust-lang#65991

`trait_upcasting` is not currently an incomplete feature; therefore examples of its use do not require `#![allow(incomplete_features)]`.

src/doc/unstable-book/src/language-features/trait-upcasting.md

@@ -12,7 +12,6 @@ so long as `Bar: Foo`.
 
 ```rust,edition2018
 #![feature(trait_upcasting)]
-#![allow(incomplete_features)]
 
 trait Foo {}