Fix imports in the book

Author: Stjepan Glavina

docs/src/concepts/futures.md

@@ -112,8 +112,7 @@ While the `Future` trait has existed in Rust for a while, it was inconvenient to
 
 ```rust,edition2018
 # extern crate async_std;
-# use async_std::{fs::File, io::Read};
-# use std::io;
+# use async_std::{fs::File, io, prelude::*};
 #
 async fn read_file(path: &str) -> Result<String, io::Error> {
     let mut file = File::open(path).await?;

docs/src/concepts/tasks.md

@@ -6,7 +6,7 @@ In `async-std`, the [`tasks`][tasks] module is responsible for this. The simples
 
 ```rust,edition2018
 # extern crate async_std;
-use async_std::{io, task, fs::File, io::Read};
+use async_std::{fs::File, io, prelude::*, task};
 
 async fn read_file(path: &str) -> Result<String, io::Error> {
     let mut file = File::open(path).await?;
@@ -33,8 +33,7 @@ This asks the runtime baked into `async_std` to execute the code that reads a fi
 
 ```rust,edition2018
 # extern crate async_std;
-# use async_std::{fs::File, io::Read, task};
-# use std::io;
+# use async_std::{fs::File, prelude::*, task};
 #
 # async fn read_file(path: &str) -> Result<String, io::Error> {
 #     let mut file = File::open(path).await?;