message: fix bert doc
diff: @@ -11,14 +11,16 @@ This example use model [BERT Chinese Model](https://www.paddlepaddle.org.cn/hubd
  Install paddlehub first
  ```
  -pip install paddlehub
  +pip3 install paddlehub
  ```
  run
  ```
  -python prepare_model.py 128
  +python3 prepare_model.py 128
  ```
  +**PaddleHub only support Python 3.5+**
  +
  the 128 in the command above means max_seq_len in BERT model, which is the length of sample after preprocessing.
  the config file and model file for server side are saved in the folder bert_seq128_model.
  the config file generated for client side is saved in the folder bert_seq128_client.
  