fix doc

deploy/cpp_infer/readme.md

@@ -119,7 +119,7 @@ tar -xf paddle_inference.tgz
 
 ```shell
 git clone https://github.com/PaddlePaddle/Paddle.git
-git checkout release/2.4
+git checkout release/2.2
 ```
 
 * 进入Paddle目录后，编译方法如下。

deploy/cpp_infer/readme_en.md

@@ -97,7 +97,7 @@ Finally you can see the following files in the folder of `paddle_inference/`.
 
 ```shell
 git clone https://github.com/PaddlePaddle/Paddle.git
-git checkout release/2.4
+git checkout release/2.2
 ```
 
 * After entering the Paddle directory, the commands to compile the paddle inference library are as follows.