Fix doctest

Author: oli-obk

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

@@ -16,13 +16,11 @@ 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(rustc_attrs)]
+#![feature(rustc_attrs, effects)]
 #![allow(internal_features)]
 
 #[rustc_intrinsic]
-fn some_intrinsic_name() -> u32 {
-    42
-}
+const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
 ```
 
 ## Intrinsics lowered to MIR instructions