diff --git a/src/doc/book/concurrency.md b/src/doc/book/concurrency.md
index 752c0972102..30e4ad7ba5b 100644
--- a/src/doc/book/concurrency.md
+++ b/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.