Fix help text being incorrect with multiple args

I'm not quite sure what the reasoning behind passing the arguments to `CmdHelp` is. Line 82 is a noop because `args` is empty. Line 78 produced odd behaviour when multiple args were passed to a non-existent command. E.g. it would print the help text for the second arg instead of the root help message:

```
$ docker fdkjhds run
Error: Command not found: fdkjhds

Usage: docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

...
```