Usage section shows Argument.Name rather than Argument.HelpName

In the help output, the Usage section shows Argument.Name, but the Arguments section shows Argument.HelpName. I think this is inconsistent and they should both use Argument.HelpName if set, so that Argument.Name would be culture-invariant and usable for System.CommandLine.NamingConventionBinder.

## Demo

```XML

Exe
net7.0
enable
enable

```

```
using System.CommandLine;
using System.IO;

var command = new RootCommand()
{
new Argument(name: "inputfile")
{
HelpName = "inputfile.log",
},
};

return await command.InvokeAsync(args);
```

```
$ dotnet run
Required argument missing for command: 'ArgumentHelpDemo'.

Description:

Usage:
ArgumentHelpDemo [options]

Arguments:

Options:
--version Show version information
-?, -h, --help Show help and usage information
```

## Current implementation

Usage section:

Arguments section: