Fix args 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 193 matching lines @@
 when you create your options.
 
 To define help text for an entire option, use the `help:` parameter:
 
     parser.addOption('mode', help: 'The compiler configuration',
         allowed: ['debug', 'release']);
     parser.addFlag('verbose', help: 'Show additional diagnostic info');
 
 For non-flag options, you can also provide a help string for the parameter:
 
-    parser.addOption('out', help: 'The output path', helpValue: 'path',
+    parser.addOption('out', help: 'The output path', valueHelp: 'path',
         allowed: ['debug', 'release']);
 
 For non-flag options, you can also provide detailed help for each expected value
 by using the `allowedHelp:` parameter:
 
     parser.addOption('arch', help: 'The architecture to compile for',
         allowedHelp: {
           'ia32': 'Intel x86',
           'arm': 'ARM Holding 32-bit chip'
         });
@@ skipping 16 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