Update the README example.
Update the README example.

This now shows up to properly construct a decoder error.

birkenfeld/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs,brandonson/docopt.rs,zsiciarz/docopt.rs,alexcrichton/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,alexcrichton/docopt.rs