fix subcommand method help

If these classes are defined,

```
module SubcommandTest1
  class Child1 < Thor
    desc "foo NAME", "Fooo"
    def foo(name)
      puts "#{name} was given"
    end
  end

  class Parent < Thor
    desc "child1", "child1 description"
    subcommand "child1", Child1
  end
end
```

`SubcommandTest1::Parent.start(%w[child1 foo])` puts messages on $stderr

on current master 
```
ERROR: "thor foo" was called with no arguments
Usage: "thor subcommand_test1:child1:foo NAME"
```

on this pull request branch
```
ERROR: "thor child1 foo" was called with no arguments
Usage: "thor child1 foo NAME"
```