Update SAVE.md

doc/SAVE.md

 ## How to save a servable model of Paddle Serving?
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+([简体中文](./SAVE_CN.md)|English)
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 - Currently, paddle serving provides a save_model interface for users to access, the interface is similar with `save_inference_model` of Paddle.
 ``` python
 import paddle_serving_client.io as serving_io