review comment: tweak error wording

Author: estebank

src/librustc_mir/borrow_check/mutability_errors.rs

@@ -49,7 +49,7 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
             (Some(desc), _) => format!("`{}`", desc),
             (None, Place::Base(PlaceBase::Local(local))) if self.mir.local_decls[*local]
                 .source_info.span.is_compiler_desugaring(CompilerDesugaringKind::Async)
-            => "async `fn` parameter".to_string(),
+            => "`async fn` parameter".to_string(),
             (None, _) => "temporary place".to_string(),
         };
         match the_place_err {

src/test/ui/async-await/issues/issue-61187.stderr

@@ -1,4 +1,4 @@
-error[E0596]: cannot borrow async `fn` parameter as mutable, as it is not declared as mutable
+error[E0596]: cannot borrow `async fn` parameter as mutable, as it is not declared as mutable
   --> $DIR/issue-61187.rs:8:5
    |
 LL | async fn response(data: Vec<u8>) {