Fix typo in readme.

pkg/args/README.md

 Parses raw command-line arguments into a set of options and values.
 
 This library supports [GNU][] and [POSIX][] style options, and it works
 in both server-side and client-side apps.
 
 ## Defining options
 
 First create an [ArgParser][]:
 
     var parser = new ArgParser();
@@ skipping 35 matching lines @@
 allowed set of values. When you do, the parser throws a [FormatException] if the
 value for an option is not in the allowed set. Here's an example of specifying
 allowed values:
 
     parser.addOption('mode', allowed: ['debug', 'release']);
 
 You can use the `callback` parameter to associate a function with an option.
 Later, when parsing occurs, the callback function is invoked with the value of
 the option:
 
-    parser.addOption('mode', callback: (mode) => print('Got mode $mode));
+    parser.addOption('mode', callback: (mode) => print('Got mode $mode'));
     parser.addFlag('verbose', callback: (verbose) {
       if (verbose) print('Verbose');
     });
 
 The callbacks for all options are called whenever a set of arguments is parsed.
 If an option isn't provided in the args, its callback is passed the default
 value, or `null` if no default value is set.
 
 ## Parsing arguments
 
@@ skipping 173 matching lines @@
 [posix]: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap12.html#tag_12_02
 [gnu]: http://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces
 [ArgParser]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser
 [ArgResults]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults
 [addOption]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addOption
 [addFlag]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addFlag
 [parse]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_parse
 [rest]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgResults#id_rest
 [addCommand]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_addCommand
 [getUsage]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/args/args.ArgParser#id_getUsage