fix lint

Author: compiler-errors

compiler/rustc_lint/src/async_fn_in_trait.rs

@@ -33,6 +33,7 @@ declare_lint! {
     ///
     /// ```rust,compile_fail
     /// # #![feature(async_fn_in_trait)]
+    /// # #![deny(async_fn_in_trait)]
     /// pub trait Trait {
     ///     async fn method(&self) {}
     /// }