Update README.md

README.md

 # Paddle Graph Learning (PGL) 
 
-[API](https://xx) | [Tutorials](https://xx)
+[DOC](https://pgl.readthedocs.io/en/latest/) | [Tutorials](https://xx)
 
 Paddle Graph Learning (PGL) is an efficient and flexible graph learning framework based on [PaddlePaddle](https://github.com/PaddlePaddle/Paddle).