📚 Documentation: Update Realtime Examples

[SiyrisSoul]

It looks like #140 didn't update the examples for Subscribe to Multiple Channels. Below is one of the examples.

documents.A should now be collections.A.documents.A

final client = Client();
final realtime = Realtime(client);
final subscription = realtime.subscribe(['documents.A', 'files']);

subscription.stream.listen((response) {
    // Callback will be executed on changes for documents A and all files.
    print(response);
})

I'm going to submit a pull request that fixes this shortly.