[react-native] Fix dev menu keyboard shortcut in sample app

Summary:
To press Cmd+Shift+Z in Sample App on the iOS Simulator is nothing happen.

To press Cmd+Control+Z is showing dev menu.
Closes react#598
Github Author: sungwook yeom <sungwook.yeom@samsung.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Author: krazyeom

Examples/SampleApp/index.ios.js

@@ -24,7 +24,7 @@ var SampleApp = React.createClass({
         </Text>
         <Text style={styles.instructions}>
           Press Cmd+R to reload,{'\n'}
-          Cmd+Shift+Z for dev menu
+          Cmd+Control+Z for dev menu
         </Text>
       </View>
     );