Improves React VR Components Sound doc

Summary:
## Motivation (required)

There was an incorrect use case.
I understand it can be use but it can be improved.
[issues-188](#188)

## Test Plan (required)

No code changes.
Closes #190

Differential Revision: D5051701

fbshipit-source-id: 3fc4520

Libraries/Sound/Sound.js

@@ -31,7 +31,7 @@ const resolveAssetSource = require('resolveAssetSource');
  * Example usage:
  * ```
  * <Image style={{height: 2.0, width: 2.0}} source={uri: 'images/waterfall.jpg'}>
- *   <Sound source={uri: 'sounds/waterfall.wav'}>
+ *   <Sound source={uri: 'sounds/waterfall.wav'} />
  * </Image>
  * ```
  *