Fix E0373 code example

Author: sledgehammervampire

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -53,7 +53,7 @@ about safety.
 
 This error may also be encountered while using `async` blocks:
 
-```compile_fail,E0373
+```compile_fail,E0373,edition2018
 use std::{sync::Arc, future::Future, pin::Pin, task::{Context, Poll}};
 
 async fn f() {