Fix typo

Summary:
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- [-] Provide a **test plan** demonstrating that the code is solid.
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What existing problem does the pull request solve?
There was a typo.

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes #14502

Differential Revision: D5245157

Pulled By: javache

fbshipit-source-id: 037e68e4ea585f73802253bb1acdd15dc38cd3b9

Author: antoinegrant

docs/GettingStarted.md

@@ -144,7 +144,7 @@ to the [Tutorial](docs/tutorial.html).
 
 Create React Native App makes it really easy to run your React Native app on a physical device without setting up a development environment. If you want to run your app on the iOS Simulator or an Android Virtual Device, please refer to the instructions for building projects with native code to learn how to install Xcode and set up your Android development environment.
 
-Once you've set these up, you can launch your app on on an Android Virtual Device by running `npm run android`, or on the iOS Simulator by running `npm run ios` (macOS only).
+Once you've set these up, you can launch your app on an Android Virtual Device by running `npm run android`, or on the iOS Simulator by running `npm run ios` (macOS only).
 
 ### Caveats