Fix warning on SectionList's tutorial

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I wanted to fix issue #16231 - the warning is not displayed anymore:

![image](https://user-images.githubusercontent.com/2101647/31268083-7e2e0ddc-aac6-11e7-966b-b1e9ffa6cfa8.png)
Closes https://github.com/facebook/react-native/pull/16232

Differential Revision: D6005386

Pulled By: hramos

fbshipit-source-id: 33961ee7cd708c424c2665a38dc5e733f1ea2204

docs/UsingAListView.md

@@ -75,6 +75,7 @@ export default class SectionListBasics extends Component {
           ]}
           renderItem={({item}) => <Text style={styles.item}>{item}</Text>}
           renderSectionHeader={({section}) => <Text style={styles.sectionHeader}>{section.title}</Text>}
+          keyExtractor={(item, index) => index}
         />
       </View>
     );