Add stability attribute to E0539 error examples

Author: unexge

src/librustc_error_codes/error_codes/E0539.md

@@ -4,6 +4,7 @@ Erroneous code example:
 
 ```compile_fail,E0539
 #![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
 
 #[rustc_deprecated(reason)] // error!
 #[unstable(feature = "deprecated_fn", issue = "123")]
@@ -27,6 +28,7 @@ To fix these issues you need to give required key-value pairs.
 
 ```
 #![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
 
 #[rustc_deprecated(since = "1.39.0", reason = "reason")] // ok!
 #[unstable(feature = "deprecated_fn", issue = "123")]