zh-CN: add the missing '[]' for 'username' (#3991)

Because 'username' is a variable instead of a property name, we cannot use it by referring with '.', a "[]" is needed here.

Author: Maledong

locale/zh-cn/docs/guides/simple-profiling.md

@@ -26,7 +26,7 @@ app.get('/newUser', (req, res) => {
 
   username = username.replace(/[!@#$%^&*]/g, '');
 
-  if (!username || !password || !users.username) {
+  if (!username || !password || !users[username]) {
     return res.sendStatus(400);
   }