change `user` to `userAdmin`

I found one typo in the topic optional chaining.

snippet:

let userAdmin = {
  admin() {
    alert("I am admin");
  }
};

In the below phrase:

Here, in both lines we first use the dot (userAdmin.admin) to get the admin property, because we assume that the userAdmin object exists, so it's safe read from it.

user should be userAdmin cause the obj's name is userAdmin not user.

PS: Thank you for these great docs! A lot of information are well cleared.

Closes: #3498

从 optional chaining 主题开始，搜索带引号的短语，并将其与附近的 userAdmin 代码片段进行比较。完成的标准是该短语始终将对象命名为 userAdmin；issue 中没有提到具体的文件或测试。