Fix some typos in tutorial (#98)

* fix typo in tutorial

* add async_std::io::BufReader to tutorial code

* writers in clean_shutdown.md return unit type

docs/src/tutorial/accept_loop.md

@@ -70,5 +70,5 @@ fn main() -> Result<()> {
 The crucial thing to realise that is in Rust, unlike other languages, calling an async function does **not** run any code.
 Async functions only construct futures, which are inert state machines.
 To start stepping through the future state-machine in an async function, you should use `.await`.
-In a non-async function, a way to execute a future is to handle it to the executor.
+In a non-async function, a way to execute a future is to hand it to the executor.
 In this case, we use `task::block_on` to execute a future on the current thread and block until it's done.

docs/src/tutorial/clean_shutdown.md

@@ -69,7 +69,7 @@ async fn broker(mut events: Receiver<Event>) -> Result<()> {
     }
     drop(peers); // 3
     for writer in writers { // 4
-        writer.await?;
+        writer.await;
     }
     Ok(())
 }

docs/src/tutorial/receiving_messages.md

@@ -10,6 +10,7 @@ We need to:
 ```rust
 use async_std::io::BufReader;
 use async_std::net::TcpStream;
+use async_std::io::BufReader;
 
 async fn server(addr: impl ToSocketAddrs) -> Result<()> {
     let listener = TcpListener::bind(addr).await?;