Fix typo (an Result)

Author: kevinastock

src/doc/book/concurrency.md

@@ -259,7 +259,7 @@ thread::spawn(move || {
 ```
 
 First, we call `lock()`, which acquires the mutex's lock. Because this may fail,
-it returns an `Result<T, E>`, and because this is just an example, we `unwrap()`
+it returns a `Result<T, E>`, and because this is just an example, we `unwrap()`
 it to get a reference to the data. Real code would have more robust error handling
 here. We're then free to mutate it, since we have the lock.