Fix 5-min doc serve command line

Downloading the latest PieCrust and trying the 5-min doc command line for serve, I get the following error message:

    kinow@chuva:/tmp/ludovicchabant-piecrust-73b63fbb517a$ bin/chef serve --root=website
    Error: Unknown option "--root".
    Type "chef --help" to get help.
    Type "chef <command> --help" to get help on specific command.

I think this PR fixes that command line. Thanks!