modify invalid url in plugins_network.md

Signed-off-by: lixiaobing10051267 <li.xiaobing1@zte.com.cn>

(cherry picked from commit fc7c235f84f76d5d64f25bc802486ecbe87f8b95)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

docs/extend/plugins_network.md

@@ -61,7 +61,7 @@ referring to that network will be sent to the plugin,
 ## Write a network plugin
 
 Network plugins implement the [Docker plugin
-API](https://docs.docker.com/extend/plugin_api/) and the network plugin protocol
+API](plugin_api.md) and the network plugin protocol
 
 ## Network plugin protocol