chore(tcp): change the hardcoded port number to port var

The `listen_on` function in the example has a `port` option but doesn't
use it

library/std/src/net/tcp.rs

@@ -869,7 +869,7 @@ pub fn incoming(&self) -> Incoming<'_> {
     /// use std::net::{TcpListener, TcpStream};
     ///
     /// fn listen_on(port: u16) -> impl Iterator<Item = TcpStream> {
-    ///     let listener = TcpListener::bind("127.0.0.1:80").unwrap();
+    ///     let listener = TcpListener::bind(format!("127.0.0.1:{port}")).unwrap();
     ///     listener.into_incoming()
     ///         .filter_map(Result::ok) /* Ignore failed connections */
     /// }