Update the `NetInfo.isConnected` example code so that it uses the new `connectionChange` event

Summary:
The first code block already uses the new `connectionChange` event instead of
the deprecated `change` event, so change this example code block as well to use
the new event.

I came across this while upgrading my RN version. In the debug-console I saw a deprecation warning, despite I was using the example-code. Looking at the source, I saw the example code block still used the deprecated event, so update it to use the new one.
Closes https://github.com/facebook/react-native/pull/16357

Differential Revision: D6054428

Pulled By: hramos

fbshipit-source-id: 72ef1a79ece7494cda3773461a740dbbdf383e7e