update install doc for release/1.8.4;test=document_fix (#26281)

README.md

@@ -33,7 +33,7 @@ pip install paddlepaddle
 # Linux GPU cuda10cudnn7
 pip install paddlepaddle-gpu
 # Linux GPU cuda9cudnn7
-pip install paddlepaddle-gpu==1.8.3.post97
+pip install paddlepaddle-gpu==1.8.4.post97
 
 ```
 It is recommended to read [this doc](https://www.paddlepaddle.org.cn/documentation/docs/en/beginners_guide/install/index_en.html) on our website.

README_cn.md

@@ -30,7 +30,7 @@ pip install paddlepaddle
 # Linux GPU cuda10cudnn7
 pip install paddlepaddle-gpu
 # Linux GPU cuda9cudnn7
-pip install paddlepaddle-gpu==1.8.3.post97
+pip install paddlepaddle-gpu==1.8.4.post97
 
 ```
 更多安装信息详见官网 [安装说明](http://www.paddlepaddle.org.cn/documentation/docs/zh/1.8/beginners_guide/install/index_cn.html)