docs: change 422 to 401

There is no need to change the status code since a validation error is still an authentication failure.
See #195 (comment)

Author: kenjis

docs/guides/mobile_apps.md

@@ -41,14 +41,14 @@ class LoginController extends BaseController
         if (! $this->validateData($this->request->getPost(), $rules)) {
             return $this->response
                 ->setJSON(['errors' => $this->validator->getErrors()])
-                ->setStatusCode(422);
+                ->setStatusCode(401);
         }
 
         // Get the credentials for login
         $credentials             = $this->request->getPost(setting('Auth.validFields'));
         $credentials             = array_filter($credentials);
         $credentials['password'] = $this->request->getPost('password');
-        
+
         // Attempt to login
         $result = auth()->attempt($credentials);
         if (! $result->isOK()) {