When using the command `/reviewers 2 reviewer` in the [PR-6752](https://github.com/openjdk/jdk/pull/6752), the bot replied the following message:

```
@DamonFool Usage: /reviewers <n> [<role>] where <n> is the number of required reviewers. If role is set, the reviewers need to have that project role. If omitted, role defaults to authors.
Number of required reviewers of role reviewers cannot be decreased below 3
```

It is wrong, and the command `/reviewers 2 reviewer` should be executed correctly.

---

Update:

I read the code in the class `ReviewersCommand` and `ReviewersTracker`. It seems the current code doesn't match the [documentation](https://wiki.openjdk.java.net/display/SKARA/Pull+Request+Commands#PullRequestCommands-/reviewers). So I would like to adjust the implementation and refactor these two classes to meet the documentation.

The original title: `The command '/reviewers 2 reviewer' can't be executed correctly`.