Use proper ES6 default export

[leonid-shevtsov]

I was getting ESLint errors on the import statement for this package. (see rule documentation.)

Technically, to use the suggested import syntax:

import PushNotificationIOS from '@react-native-community/push-notification-ios';

the package should have an export default.

And since you are already using ES6 import statements, it won't harm compatibility.

[Naturalclar]

thanks!