update tun2 example (#42)

* update tun2 example

* cargo fmt

examples/tun2.rs

@@ -78,12 +78,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let ipv4 = Ipv4Addr::new(10, 0, 0, 33);
     let netmask = Ipv4Addr::new(255, 255, 255, 0);
-    let _gateway = Ipv4Addr::new(10, 0, 0, 1);
+    #[cfg(not(target_os = "windows"))]
+    let gateway = Ipv4Addr::new(10, 0, 0, 1);
 
     let mut tun_config = tun2::Configuration::default();
     tun_config.address(ipv4).netmask(netmask).mtu(MTU).up();
     #[cfg(not(target_os = "windows"))]
-    tun_config.destination(_gateway); // avoid routing all traffic to tun on Windows platform
+    tun_config.destination(gateway); // avoid routing all traffic to tun on Windows platform
 
     #[cfg(target_os = "linux")]
     tun_config.platform_config(|p_cfg| {