Fix typo in readme.

pkg/args/README.md

Index: pkg/args/README.md
diff --git a/pkg/args/README.md b/pkg/args/README.md
index fe31554cba93d95e5f433ba50a3ac5a69e93fe6d..6e36435edf31208e43919ee8208a0eb6d8c5f87d 100644
--- a/pkg/args/README.md
+++ b/pkg/args/README.md
@@ -53,7 +53,7 @@ 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');
     });