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

從 optional chaining 主題開始，搜尋加上引號的片語，並將其與附近的 userAdmin 程式碼片段進行比較。完成的標準是該片語始終將物件命名為 userAdmin；issue 中沒有提到特定的檔案或測試。