第二版的第二章的jupyter版本的图片路径不对

the picture address in ```Chapter2.md``` is like 
```![图2-3  Windows下使用pip安装PyTorch](imgs/pip_PyTorch_Windows.png)```
but in ```Chapter2.ipynb``` is 
```![图2-3  使用pip安装PyTorch(Windows)](imgs/使用pip安装PyTorch(Windows).png) ```.
