attr: fix expected meta-item for #[stable]

When an unexpected meta item is provided to `#[stable]`, the diagnostic
lists "since" and "note" as expected meta-items, however the surrounding
code actually expects "feature" and "since".

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_attr/src/builtin.rs

@@ -404,7 +404,7 @@ where
                                         meta.span(),
                                         AttrError::UnknownMetaItem(
                                             pprust::path_to_string(&mi.path),
-                                            &["since", "note"],
+                                            &["feature", "since"],
                                         ),
                                     );
                                     continue 'outer;