fix: 修复示例错误

[DouMJun]

原文：

if it's user names you can do { [username:string] : {message: string} } to help the next dev who looks at the code (which just might happen to be you)

本文：

如果有一个用户名，你可以使用 { username: string}: { message: string }，这有利于下一个开发者理解你的代码。