fix: add missing underscore

Author: krushnarout

src/content/4/en/part4c.md

@@ -438,7 +438,7 @@ notesRouter.post('/', async (request, response) => {
   const note = new Note({
     content: body.content,
     important: body.important === undefined ? false : body.important,
-    user: user.id //highlight-line
+    user: user._id //highlight-line
   })
 
   const savedNote = await note.save()