Fix how react is imported

Summary:
Thanks for submitting a pull request! Please provide enough information so that others can review your pull request:

(You can skip this if you're fixing a typo or adding an app to the Showcase.)

Explain the **motivation** for making this change. What existing problem does the pull request solve?

Prefer **small pull requests**. These are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

**Test plan (required)**

Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI.

Make sure tests pass on both Travis and Circle CI.

**Code formatting**

Look around. Match the style of the rest of the codebase. See also the simple [style guide](https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#style-guide).

For more info, see the ["Pull Requests" section of our "Contributing" guidelines](https://github.com/facebook/react-native/blob/mas
Closes https://github.com/facebook/react-native/pull/7962

Differential Revision: D3397289

fbshipit-source-id: 21adf955af4a623f1ce71e7a5e412020ceaad12a

docs/Tutorial-CoreComponents.md

@@ -88,7 +88,8 @@ Direct text-based user input is a foundation for many apps. Writing a post or co
 This example creates a simple `TextInput` box with the `string` `Hello` as the placeholder when the `TextInput` is empty.
 
 ```JavaScript
-import React, { AppRegistry, TextInput, View } from 'react-native'
+import React from 'react';
+import { AppRegistry, TextInput, View } from 'react-native';
 
 const App = () => {
   return (