doc: make TcpListener example more simple

src/libstd/net/tcp.rs

@@ -45,7 +45,6 @@ pub struct TcpStream(net_imp::TcpStream);
 ///
 /// ```no_run
 /// use std::net::{TcpListener, TcpStream};
-/// use std::thread;
 ///
 /// let listener = TcpListener::bind("127.0.0.1:80").unwrap();
 ///
@@ -57,17 +56,11 @@ pub struct TcpStream(net_imp::TcpStream);
 /// for stream in listener.incoming() {
 ///     match stream {
 ///         Ok(stream) => {
-///             thread::spawn(move|| {
-///                 // connection succeeded
-///                 handle_client(stream)
-///             });
+///             handle_client(stream);
 ///         }
 ///         Err(e) => { /* connection failed */ }
 ///     }
 /// }
-///
-/// // close the socket server
-/// drop(listener);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct TcpListener(net_imp::TcpListener);