updated README to reflect new signiture for onPrev and onNext
updated README to reflect new signiture for onPrev and onNext

cyprusglobe/react-native-navbar,christopherdro/react-native-navbar,Kureev/react-native-navbar,kietsy/react-native-navbar,react-native-fellowship/react-native-navbar,ethaizone/react-native-navbar,idibidiart/react-native-navbar,umfana/react-native-navbar,APSL/react-native-navbar