Small technical correction. (#64)

Absolutely awesome work here! Just reading through the book absorbing all of the awesomeness and noticed this little bit. Let me know if you would like for me to update the corrections in any way.

Author: thedodd

Diff for: docs/src/concepts/futures.md

@@ -102,7 +102,7 @@ While the `Future` trait has existed in Rust for a while, it was inconvenient to
 
 Amazingly little difference, right? All we did is label the function `async` and insert 2 special commands: `.await`.
 
-This function sets up a deferred computation. When this function is called, it will produce a `Future<Output=String>` instead of immediately returning a String. (Or, more precisely, generate a type for you that implements `Future<Output=String>`.)
+This function sets up a deferred computation. When this function is called, it will produce a `Future<Output=Result<String, io::Error>>` instead of immediately returning the `Result`. (Or, more precisely, generate a type for you that implements `Future<Output=Result<String, io::Error>>`.)
 
 ## What does `.await` do?