Added styles.viewPager to complete example code

Summary:
Otherwise, you're met with a bewildering blank page. :D

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Closes facebook/react-native#15754

Differential Revision: D5768992

Pulled By: hramos

fbshipit-source-id: 39a9f7c208d635e089751015dcf2536144ec0176

ViewPagerAndroid.android.js

@@ -61,6 +61,9 @@ export type ViewPagerScrollState = $Enum<{
  *
  * var styles = {
  *   ...
+ *   viewPager: {
+ *     flex: 1
+ *   },
  *   pageStyle: {
  *     alignItems: 'center',
  *     padding: 20,