Fix broken link to install paddlepaddle
The link provided for installing paddlepaddle doesn't work. Hence, this change updates the broken link to install paddlepaddle for CPU and GPU.
- If you have CUDA 9 or CUDA 10 installed on your machine, please run the following command to install
```bash
- python3 -m pip install paddlepaddle-gpu -i https://mirror.baidu.com/pypi/simple
+ python -m pip install paddlepaddle-gpu -i https://pypi.tuna.tsinghua.edu.cn/simple
```
- If you have no available GPU on your machine, please run the following command to install the CPU version
```bash
- python3 -m pip install paddlepaddle -i https://mirror.baidu.com/pypi/simple
+ python -m pip install paddlepaddle -i https://pypi.tuna.tsinghua.edu.cn/simple
```
For more software version requirements, please refer to the instructions in [Installation Document](https://www.paddlepaddle.org.cn/install/quick) for operation.
