Fix unhelpful help output

[Release-Candidate]

resyntax --help does not print available commands (fix and analyze). Add this information and a link to the Resyntax documentation.

New output of resyntax --help:

usage: resyntax [ <option> ... ] <command> [<leftover-args>] ...

<command> is one of

        analyze
        fix

For help on these, use 'analyze --help' or 'fix --help'.

<option> is one of

  --help, -h
     Show this help
  --
     Do not treat any remaining argument as a switch (at this level)

 Multiple single-letter switches can be combined after
 one `-`. For example, `-h-` is the same as `-h --`.

See https://docs.racket-lang.org/resyntax/index.html for details.

[jackfirth]

Thank you very much!