Update error message in test

Author: varkor

src/test/ui/feature-gates/feature-gate-async-await.stderr

@@ -51,5 +51,4 @@ LL |     let _ = async || {};
 
 error: aborting due to 6 previous errors
 
-Some errors occurred: E0658, E0706.
-For more information about an error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0658`.