docs: update report the port (#573)

Author: Kikobeats

README.md

@@ -26,7 +26,7 @@ const ProxyChain = require('proxy-chain');
 const server = new ProxyChain.Server({ port: 8000 });
 
 server.listen(() => {
-    console.log(`Proxy server is listening on port ${8000}`);
+    console.log(`Proxy server is listening on port ${server.port}`);
 });
 ```