remove `subcommand --help` output comma when more than one arg

supose I have defined a program `app` below
```js
var program = require('commander');

program
    .version('0.0.1');

program
    .command('exec <arg1> [arg2]')
    .description('exec example')
    .action(function(arg1, arg2, options){
        console.log('called exec');
    });

program.parse(process.argv);
```

Then when I get the exec help info via `app exec --help`, it will output 
```bash
Usage: exec [options] <arg1>,[arg2]

  Options:

    -h, --help  output usage information
```

But the right output should be
```bash
Usage: exec [options] <arg1> [arg2]

  Options:

    -h, --help  output usage information
```

So, I find the problem is that the `args` is an array, when its length is larger than 1, it will join with `,`, what I do is join the array with space :)