FIx doctest

Author: oli-obk

src/doc/unstable-book/src/language-features/intrinsics.md

@@ -16,7 +16,8 @@ some features that only exist on some backends. Backends can simply not implemen
 intrinsics without causing any code miscompilations or failures to compile.
 
 ```rust
-#![feature(core_intrinsics)]
+#![feature(rustc_attrs)]
+#![allow(internal_features)]
 
 #[rustc_intrinsic]
 fn some_intrinsic_name() -> u32 {