correct the script for installing mmcv-full (#399)

Author: 钟晓键

docs/get_started.md

@@ -35,7 +35,7 @@ Either `mmcv` or `mmcv-full` is compatible with MMSegmentation, but for methods
 The pre-build mmcv-full (with PyTorch 1.5 and CUDA 10.1) can be installed by running: (other available versions could be found [here](https://mmcv.readthedocs.io/en/latest/#install-with-pip))
 
 ```shell
-pip install mmcv-full==latest+torch1.5.0+cu101 -f https://download.openmmlab.com/mmcv/dist/index.html
+pip install mmcv-full -f https://download.openmmlab.com/mmcv/dist/cu101/torch1.5.0/index.html
 ```
 
 **Install mmcv for Windows (Experimental):**