Update README.md

Change installation command from `pip install -e .` to `pip install .`. Otherwise Texar-PyTorch won't work well with Texar-TF.

README.md

@@ -95,7 +95,7 @@ After PyTorch is installed, please run the following commands to install Texar-P
 ```
 git clone https://github.com/asyml/texar-pytorch.git 
 cd texar-pytorch
-pip install -e .
+pip install .
 ```
 
 To use tensorboard support with `Executor`, please install `tensorboardX` with the following command