Add missing word

This PR adds a missing "how" to the description of the iteration helpers' behavior. 

"Each of these helpers accepts a function callback to apply to each element in the array, differing only in *how* they respectively respond to a return value from the callback."

this & object prototypes/ch3.md

@@ -813,7 +813,7 @@ for (var i = 0; i < myArray.length; i++) {
 
 This isn't iterating over the values, though, but iterating over the indices, where you then use the index to reference the value, as `myArray[i]`.
 
-ES5 also added several iteration helpers for arrays, including `forEach(..)`, `every(..)`, and `some(..)`. Each of these helpers accepts a function callback to apply to each element in the array, differing only in they respectively respond to a return value from the callback.
+ES5 also added several iteration helpers for arrays, including `forEach(..)`, `every(..)`, and `some(..)`. Each of these helpers accepts a function callback to apply to each element in the array, differing only in how they respectively respond to a return value from the callback.
 
 `forEach(..)` will iterate over all values in the array, and ignores any callback return values. `every(..)` keeps going until the end *or* the callback returns a `false` (or "falsy") value, whereas `some(..)` keeps going until the end *or* the callback returns a `true` (or "truthy") value.