Mark E0802 examples as 2018 edition

Author: cramertj

compiler/rustc_error_codes/src/error_codes/E0802.md

@@ -2,7 +2,7 @@ The `Async{Fn, FnMut, FnOnce}` traits are not yet dyn compatible.
 
 Erroneous code example:
 
-```compile_fail,E0802
+```compile_fail,E0802,edition2018
 async fn call_async_fn_twice(some_fn: &dyn AsyncFn()) {
   some_fn().await;
   some_fn().await;
@@ -11,7 +11,7 @@ async fn call_async_fn_twice(some_fn: &dyn AsyncFn()) {
 
 One workaround to this issue is to use `impl Async...` instead:
 
-```
+```edition2018
 async fn call_async_fn_twice(some_fn: &impl AsyncFn()) {
   some_fn().await;
   some_fn().await;