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