Avoid doctest errors

Author: kornelski

Diff for: src/lib.rs

@@ -138,7 +138,7 @@
 //!
 //! Call an async function from the main function:
 //!
-//! ```
+//! ```rust,ignore
 //! async fn say_hello() {
 //!     println!("Hello, world!");
 //! }
@@ -151,7 +151,7 @@
 //!
 //! Await two futures concurrently, and return a tuple of their output:
 //!
-//! ```
+//! ```rust,ignore
 //! use async_std::prelude::*;
 //!
 //! #[async_std::main]
@@ -164,7 +164,7 @@
 //!
 //! Create a UDP server that echoes back each received message to the sender:
 //!
-//! ```no_run
+//! ```no_run,ignore
 //! use async_std::net::UdpSocket;
 //!
 //! #[async_std::main]