Change "dartfmt" -> "dart format" in test generator.

Change-Id: I1da2fc67dcfa1502b63b2cdbb7e460a86ac22967
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215159
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Kevin Moore <kevmoo@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>

tools/test_generators/async_nested_test_generator.dart

@@ -39,7 +39,7 @@ main(List<String> args) async {
         String name = "async_nested_${maxSize}_${i}_test.dart";
         asyncFile.renameSync(name);
         print(" -> Created $name");
-        print("    (you might want to run dartfmt -w $name).");
+        print("    (you might want to run 'dart format $name').");
       }
     }
   }