Update part4d.md

error code corrected and changed error message to match line 230

Author: myverdict

src/content/4/en/part4d.md

@@ -227,7 +227,7 @@ const errorHandler = (error, request, response, next) => {
   } else if (error.name === 'MongoServerError' && error.message.includes('E11000 duplicate key error')) {
     return response.status(400).json({ error: 'expected `username` to be unique' })
   } else if (error.name ===  'JsonWebTokenError') { // highlight-line
-    return response.status(400).json({ error: 'token missing or invalid' }) // highlight-line
+    return response.status(401).json({ error: 'token invalid' }) // highlight-line
   }
 
   next(error)