Update ios-guide.md (#538)
Update ios-guide.md (#538)

* Update ios-guide.md

Added not-deprecated openURL and proper header reference

* Update ios-guide.md

devfd/react-native-google-signin,apptailor/react-native-google-signin,devfd/react-native-google-signin,devfd/react-native-google-signin,apptailor/react-native-google-signin