Fix Ray Serve example (ray-project#24276)

The current example in the `README.md` is broken, which is same issue as ray-project#22228. cc @simon-mo

README.rst

@@ -338,7 +338,7 @@ This example runs serves a scikit-learn gradient boosting classifier.
  self.label_list = iris_dataset["target_names"].tolist()
 
  async def __call__(self, request):
- payload = await request.json()["vector"]
+ payload = (await request.json())["vector"]
  print(f"Received flask request with data {payload}")
 
  prediction = self.model.predict([payload])[0]