rust-lang#36553 specialisation error 502 is misleading

this commit correct E0502 error text.
See referenced issue for further info

Author: giannicic

src/librustc_typeck/check/mod.rs

@@ -904,15 +904,15 @@ fn report_forbidden_specialization<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let mut err = struct_span_err!(
         tcx.sess, impl_item.span, E0520,
         "`{}` specializes an item from a parent `impl`, but \
-         neither that item nor the `impl` are marked `default`",
+         that item is not marked `default`",
         impl_item.name);
     err.span_label(impl_item.span, &format!("cannot specialize default item `{}`",
                                             impl_item.name));
 
     match tcx.span_of_impl(parent_impl) {
         Ok(span) => {
             err.span_label(span, &"parent `impl` is here");
-            err.note(&format!("to specialize, either the parent `impl` or `{}` \
+            err.note(&format!("to specialize, `{}` \
                                in the parent `impl` must be marked `default`",
                               impl_item.name));
         }

src/test/compile-fail/E0520.rs

@@ -27,7 +27,7 @@ impl SpaceLlama for i32 {
     default fn fly(&self) {}
     //~^ ERROR E0520
     //~| NOTE cannot specialize default item `fly`
-    //~| NOTE either the parent `impl` or `fly` in the parent `impl` must be marked `default`
+    //~| NOTE `fly` in the parent `impl` must be marked `default`
 }
 
 fn main() {