update README.md to 3.23.08

Author: wenyh1

README.md

@@ -10,7 +10,7 @@
 [下载](https://github.com/actiontech/dble/releases) 或者[github镜像站下载](https://hub.fastgit.org/actiontech/dble/releases)
 
 ## 文档PDF下载
-[文档PDF下载](https://github.com/actiontech/dble-docs-cn/releases/download/3.23.04.0%2Ftag/dble-manual-3.23.04.0.pdf)  或者[github镜像站下载](https://hub.fastgit.org/actiontech/dble-docs-cn/releases/download/3.23.04.0%2Ftag/dble-manual-3.23.04.0.pdf)     
+[文档PDF下载](https://github.com/actiontech/dble-docs-cn/releases/download/3.23.08.0%2Ftag/dble-manual-3.23.08.0.pdf)  或者[github镜像站下载](https://hub.fastgit.org/actiontech/dble-docs-cn/releases/download/3.23.08.0%2Ftag/dble-manual-3.23.08.0.pdf)     
 注意：如版本尚未发布，则不能下载