Reworded sentence about showAsAction in topbar button doc (#5147)
Reworded sentence about showAsAction in topbar button doc (#5147)

Fixed minor linguistic mistake

guyca/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,guyca/react-native-navigation,thanhzusu/react-native-navigation,thanhzusu/react-native-navigation,3sidedcube/react-native-navigation,thanhzusu/react-native-navigation,wix/react-native-navigation,3sidedcube/react-native-navigation,wix/react-native-navigation,wix/react-native-navigation,guyca/react-native-navigation,guyca/react-native-navigation,thanhzusu/react-native-navigation,wix/react-native-navigation,thanhzusu/react-native-navigation,3sidedcube/react-native-navigation,thanhzusu/react-native-navigation