Visibly import in tasks example

Fixes #97

Author: skade

docs/src/concepts/tasks.md

@@ -6,9 +6,8 @@ In `async-std`, the [`tasks`][tasks] module is responsible for this. The simples
 
 ```rust,edition2018
 # extern crate async_std;
-# use async_std::{fs::File, io::Read, task};
-# use std::io;
-#
+use async_std::{io, task, fs::File, io::Read};
+
 async fn read_file(path: &str) -> Result<String, io::Error> {
     let mut file = File::open(path).await?;
     let mut contents = String::new();