doc: update docs/dart.md (#814)

### Current behavior
The current asynchronous login sample code has the following issues:
1. The `main` function tries to use `await userName()`, but `userName` is a string variable, not a function.
2. the `login` function should be called and `await` should be used to wait for its result.
### Expected behavior
The `main` function should correctly call the `login` function and use `await` to wait for the result, then print out the username.
### Sample code
Here is the corrected code:
```dart
Future<String> login() {
String userName = "Temidjoy";
return Future.delayed(
Duration(seconds: 4), () => userName
).
}
main() async {
print('Authenticating please wait...') The
String result = await login();
String result = await login(); print(result);
}

docs/dart.md

@@ -842,7 +842,8 @@ Future<String> login() {
 // 异步
 main() async {
   print('Authenticating please wait...');
-  print(await userName());
+  String result = await login();
+  print(result);
 }
 ```