pkg/args: Fixed missing comma in text and updated README syntax

test/args_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library args_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 import 'utils.dart';
 
@@ skipping 284 matching lines @@
   'with\ttab',
   'with\rcarriage\rreturn',
   'with\nline\nfeed',
   "'singlequotes'",
   '"doublequotes"',
   'back\\slash',
   'forward/slash'
 ];
 
 const _VALID_OPTIONS = const [
-  'a' // One character.
-      'contains-dash',
+  'a', // One character.
+  'contains-dash',
   'contains_underscore',
   'ends-with-dash-',
   'contains--doubledash--',
   '1starts-with-number',
   'contains-a-1number',
   'ends-with-a-number8'
 ];