message: change ocr model download url
diff: ## Get Model
  ```
  -python3 -m paddle_serving_app.package --get_model ocr_rec
  +wget https://paddle-serving.bj.bcebos.com/paddle_hub_models/image/OCR/ocr_rec.tar.gz
  tar -xzvf ocr_rec.tar.gz
  -python3 -m paddle_serving_app.package --get_model ocr_det
  +wget https://paddle-serving.bj.bcebos.com/ocr/ocr_det.tar.gz
  tar -xzvf ocr_det.tar.gz
  ```
  