opt cpp_infer doc

deploy/cpp_infer/readme.md

@@ -18,6 +18,7 @@ PaddleOCR模型部署。
 * 首先需要从opencv官网上下载在Linux环境下源码编译的包，以opencv3.4.7为例，下载命令如下。
 
 ```
+cd deploy/cpp_infer
 wget https://github.com/opencv/opencv/archive/3.4.7.tar.gz
 tar -xf 3.4.7.tar.gz
 ```

deploy/cpp_infer/readme_en.md

@@ -18,6 +18,7 @@ PaddleOCR model deployment.
 * First of all, you need to download the source code compiled package in the Linux environment from the opencv official website. Taking opencv3.4.7 as an example, the download command is as follows.
 
 ```
+cd deploy/cpp_infer
 wget https://github.com/opencv/opencv/archive/3.4.7.tar.gz
 tar -xf 3.4.7.tar.gz
 ```