mediaPlaybackRequiresUserAction default value is true,not `false`

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

> **Unless you are a React Native release maintainer and cherry-picking an *existing* commit into a current release, ensure your pull request is targeting the `master` React Native branch.**

(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/face
Closes https://github.com/facebook/react-native/pull/8999

Differential Revision: D3664512

Pulled By: javache

fbshipit-source-id: 030fe4752e53efcde21baadfc2502413b7fc9c8c

Libraries/Components/WebView/WebView.ios.js

@@ -321,7 +321,7 @@ class WebView extends React.Component {
 
     /**
      * Boolean that determines whether HTML5 audio and video requires the user
-     * to tap them before they start playing. The default value is `false`.
+     * to tap them before they start playing. The default value is `true`.
      */
     mediaPlaybackRequiresUserAction: PropTypes.bool,
   };