fix typo in simple-profiling.md (#4115)

Check if the user name exists, "400" will be back.

Author: meixg

locale/en/docs/guides/simple-profiling.md

@@ -59,7 +59,7 @@ app.get('/auth', (req, res) => {
 
   username = username.replace(/[!@#$%^&*]/g, '');
 
-  if (!username || !password || users[username]) {
+  if (!username || !password || !users[username]) {
     return res.sendStatus(400);
   }