Summary:
The list items' selected state is inside the `selected` Map, not inside the `state`.
This PR corrects a small mistake in the minimal example in the Documentatioin of the FlatList component.
1. Create a vanilla React Native project.
2. Create the components involved in the minimal example of FlatList.
3. Run to see if the `selected` property of the `MyListItem` changes as intended.
Currently the example has this mistake so an error will show up when running:
Closes https://github.com/facebook/react-native/pull/13795
Differential Revision: D5010105
Pulled By: javache
fbshipit-source-id: 09585cea2f2e3e6746419ef54ef8da9dbdb8dbb1
