fixes #1803.

docs/guide/form.md

@@ -20,7 +20,7 @@ class LoginForm extends Model
 	{
 		return [
 			// username and password are both required
-			['username, password', 'required'],
+			[['username', 'password'], 'required'],
 			// password is validated by validatePassword()
 			['password', 'validatePassword'],
 		];