Generate Help on a specific verb

**Describe the bug**
When I call my program with the following arguments : "help verb1", I expect that the help for this specific verb was displayed. But instead, the help for all verbs are displayed.

**To Reproduce**
```
[Verb("Verb1", HelpText ="Help Verb1")]
internal class Verb1
{
[Option(Required = true, HelpText = "Help MyOption1")]
public string MyOption1 { get; set; }
}

[Verb("Verb2",HelpText = "Help Verb2")]
internal class Verb2
{
[Option(Required = true, HelpText = "Help MyOption2")]
public string MyOption2 { get; set; }
}

internal class Program
{
static void Main(string[] args)
{
var parser = new Parser(config => config.HelpWriter = null);
var parserResult = parser.ParseArguments<
Verb1,
Verb2
>
(args);

parserResult.MapResult(
(Verb1 options) => { return 0; },
(Verb2 options) => { return 0; },
errors => {
DisplayHelp(parserResult, errors);
return 0;
});

Console.ReadKey();
}

static void DisplayHelp(ParserResult result, IEnumerable errs)
{
var helpText = HelpText.AutoBuild(result, h =>
{
h.AdditionalNewLineAfterOption = false;
h.Heading = "Myapp 2.0.0-beta"; //change header
h.Copyright = "Copyright (c) 2019 Global.com"; //change copyright text
return HelpText.DefaultParsingErrorsHandler(result, h);
}, e => e, verbsIndex:true);
Console.WriteLine(helpText);
}
}
```
When I excecute this with the command line "MyApp.exe help verb1"
The actual result is :

**Expected behavior**
This display :
Verb2 Help Verb1
MyOption2 Help MyOption2

使用所示的 Verb1 和 Verb2 类复现该问题，然后跟踪结果从 ParseArguments 经过 DisplayHelp、HelpText.AutoBuild 和 DefaultParsingErrorsHandler 的处理过程。检查如何解释 "help verb1" 的参数，以及 verbsIndex 如何影响输出。完成标准是：请求某个 verb 的帮助时，只显示该 verb 的帮助和选项，同时保留现有的所有 verb 帮助行为。

由索引模型根据 Issue 内容生成。