Update FlatList.js

Summary:
Only a small amendment, but took me a little bit to figure out why this wasn't working.
Closes https://github.com/facebook/react-native/pull/12706

Differential Revision: D4656700

fbshipit-source-id: d6038581aa70e96a2be775a7a9786e8c7e64f762

Libraries/CustomComponents/Lists/FlatList.js

@@ -51,7 +51,7 @@ type RequiredProps<ItemT> = {
    *     _renderItem = ({item}) => (
    *       <TouchableOpacity onPress={() => this._onPress(item)}>
    *         <Text>{item.title}}</Text>
-   *       <TouchableOpacity/>
+   *       </TouchableOpacity>
    *     );
    *     ...
    *     <FlatList data={[{title: 'Title Text', key: 'item1'}]} renderItem={this._renderItem} />