Update osa4.md

All other examples use res.json()

osa4.md

@@ -2254,7 +2254,7 @@ loginRouter.post('/', async (request, response) => {
     await bcrypt.compare(body.password, user.passwordHash)
 
   if ( !(user && passwordCorrect) ) {
-    return response.status(401).send({ error: 'invalid username or password' })
+    return response.status(401).json({ error: 'invalid username or password' })
   }
 
   const userForToken = {