Better help message. (flutter#9097)

Author: kangwang1988

tools/gn

@@ -264,7 +264,7 @@ def parse_args(args):
   parser.add_argument('--dynamic', default=False, action='store_true')
   parser.add_argument('--interpreter', default=False, action='store_true')
   parser.add_argument('--dart-debug', default=False, action='store_true', help='Enables assertsion in the Dart VM. ' +
-      'Does not affect affect optimization levels. If you need to disable optimizations in Dart, use --full-dart-debug')
+      'Does not affect optimization levels. If you need to disable optimizations in Dart, use --full-dart-debug')
   parser.add_argument('--full-dart-debug', default=False, action='store_true', help='Implies --dart-debug ' +
       'and also disables optimizations in the Dart VM making it easier to step through VM code in the debugger.')