explicitly show how to add the optional listener

Summary:
The previous example only showed where to add the optional "listener" but didn't show how to make use of it.

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Closes https://github.com/facebook/react-native/pull/15576

Differential Revision: D5911716

Pulled By: shergin

fbshipit-source-id: 60023470d23c2cbbde47ab9aa82c7ecef73be467

Libraries/Animated/src/AnimatedImplementation.js

@@ -839,12 +839,13 @@ module.exports = {
    *```javascript
    *  onScroll={Animated.event(
    *    [{nativeEvent: {contentOffset: {x: this._scrollX}}}],
-   *    {listener},          // Optional async listener
+   *    {listener: (event) => console.log(event)}, // Optional async listener
    *  )}
    *  ...
    *  onPanResponderMove: Animated.event([
    *    null,                // raw event arg ignored
    *    {dx: this._panX},    // gestureState arg
+        {listener: (event, gestureState) => console.log(event, gestureState)}, // Optional async listener
    *  ]),
    *```
    *