clarify the picker onValueChange

Summary:
In the current docs, it's not quite clear, at the first sight, what the `lang` parameter passed to `onValueChange` is. This makes it obvious.
Closes https://github.com/facebook/react-native/pull/13617

Differential Revision: D4969592

Pulled By: javache

fbshipit-source-id: e94bedefebbe19d838d010f0c79d9a76743a4341

Libraries/Components/Picker/Picker.js

@@ -77,7 +77,7 @@ class PickerItem extends React.Component {
  *
  *     <Picker
  *       selectedValue={this.state.language}
- *       onValueChange={(lang) => this.setState({language: lang})}>
+ *       onValueChange={(itemValue, itemIndex) => this.setState({language: itemValue})}>
  *       <Picker.Item label="Java" value="java" />
  *       <Picker.Item label="JavaScript" value="js" />
  *     </Picker>