Update Touchable docs with new image require syntax

Summary:
Just noticed the old syntax was used in the docs.
Closes https://github.com/facebook/react-native/pull/8765

Differential Revision: D3560355

Pulled By: mkonicek

fbshipit-source-id: 13db21a498be205a32921a89058167dd47e1bbdf

Libraries/Components/Touchable/TouchableHighlight.js

@@ -54,7 +54,7 @@ var PRESS_RETENTION_OFFSET = {top: 20, left: 20, right: 20, bottom: 30};
  *     <TouchableHighlight onPress={this._onPressButton}>
  *       <Image
  *         style={styles.button}
- *         source={require('image!myButton')}
+ *         source={require('./myButton.png')}
  *       />
  *     </TouchableHighlight>
  *   );

Libraries/Components/Touchable/TouchableOpacity.js

@@ -41,7 +41,7 @@ var PRESS_RETENTION_OFFSET = {top: 20, left: 20, right: 20, bottom: 30};
  *     <TouchableOpacity onPress={this._onPressButton}>
  *       <Image
  *         style={styles.button}
- *         source={require('image!myButton')}
+ *         source={require('./myButton.png')}
  *       />
  *     </TouchableOpacity>
  *   );