Update RNExitApp.mm

[kr-yeon]

Apps that look like errors may be rejected by Apple. So I corrected it.

We found that your app includes a UI control for quitting the app. This is not in compliance with the iOS Human Interface Guidelines, as required by the App Store Review Guidelines

[wumke]

I never created an app with a UI control for quitting the app, only used it to kill the app after a visual warning that rights must be granted for the app to work correctly (which was also mentionned in the app description). If users do not grant the requested rights, my app cannot work. That app never got rejected.

Triggering the suspend does not completely kill the app, if I remember correctly. There has already been a request for 'gracefully close the app', see #62 . This makes sense... but I would like to keep a hard kill as standard in any case, altough an additional method 'suspendApp' would be a nice addition...

I also want to keep functionalty on both platforms as identical as possible, so if you can send a pull request for suspending on both platforms, with an additional method 'suspendApp', I'll merge that!