Update Installing.md (#3871)
Update Installing.md (#3871)

Change `index.ios` to `index` in `AppDelegate.m`.
wix/react-native-navigation,Jpoliachik/react-native-navigation,ceyhuno/react-native-navigation,Jpoliachik/react-native-navigation,ceyhuno/react-native-navigation,Jpoliachik/react-native-navigation,chicojasl/react-native-navigation,guyca/react-native-navigation,wix/react-native-navigation,chicojasl/react-native-navigation,guyca/react-native-navigation,wix/react-native-navigation,chicojasl/react-native-navigation,ceyhuno/react-native-navigation,thanhzusu/react-native-navigation,thanhzusu/react-native-navigation,3sidedcube/react-native-navigation,3sidedcube/react-native-navigation,3sidedcube/react-native-navigation,thanhzusu/react-native-navigation,wix/react-native-navigation,ceyhuno/react-native-navigation,guyca/react-native-navigation,chicojasl/react-native-navigation,Jpoliachik/react-native-navigation,thanhzusu/react-native-navigation,thanhzusu/react-native-navigation,3sidedcube/react-native-navigation,guyca/react-native-navigation,Jpoliachik/react-native-navigation,wix/react-native-navigation,ceyhuno/react-native-navigation,ceyhuno/react-native-navigation,chicojasl/react-native-navigation,Jpoliachik/react-native-navigation,chicojasl/react-native-navigation,wix/react-native-navigation,thanhzusu/react-native-navigation