Fix typo (fixes nzakas#346)

Author: nzakas

manuscript/B-ECMAScript-7.md

@@ -98,7 +98,7 @@ console.log(values.indexOf(NaN));       // -1
 console.log(values.includes(NaN));      // true
 ```
 
-The `values.indexOf()` method returns `-1` for `NaN` even though `NaN` is contained in the `values` array. On the other hand, `values.includes()` returns `true` for `NaN` because it uses a different value comparison operator used.
+The `values.indexOf()` method returns `-1` for `NaN` even though `NaN` is contained in the `values` array. On the other hand, `values.includes()` returns `true` for `NaN` because it uses a different value comparison operator.
 
 W> When you want to check just for the existence of a value in an array and don't need to know the index , I recommend using `includes()` because of the difference in how `NaN` is treated by the `includes()` and `indexOf()` methods. If you do need to know where in the array a value exists, then you have to use the `indexOf()` method.