Update stderr file

src/test/ui/underscore_const_names_feature_gate.stderr

 error[E0658]: naming constants with `_` is unstable (see issue #54912)
   --> $DIR/underscore_const_names_feature_gate.rs:11:1
    |
-LL | const _: () = ();
+LL | const _: () = (); //~ ERROR is unstable
    | ^^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(underscore_const_names)] to the crate attributes to enable
@@ -9,7 +9,7 @@ LL | const _: () = ();
 error[E0658]: naming constants with `_` is unstable (see issue #54912)
   --> $DIR/underscore_const_names_feature_gate.rs:12:1
    |
-LL | static _: () = ();
+LL | static _: () = (); //~ ERROR is unstable
    | ^^^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(underscore_const_names)] to the crate attributes to enable