Update code samples in Stack Navigator docs to correctly use defaultNavigationOptions in navigator config

Author: levibuzolic

docs/stack-navigator.md

@@ -31,7 +31,7 @@ createStackNavigator({
     // The action and route params are extracted from the path.
 
     // Optional: Override the `navigationOptions` for the screen
-    navigationOptions: ({ navigation }) => ({
+    defaultNavigationOptions: ({ navigation }) => ({
       title: `${navigation.state.params.name}'s Profile'`,
     }),
   },
@@ -261,7 +261,7 @@ const ModalNavigator = createStackNavigator(
   {
     headerMode: 'none',
     mode: 'modal',
-    navigationOptions: {
+    defaultNavigationOptions: {
       gesturesEnabled: false,
     },
     transitionConfig: () => ({
@@ -321,4 +321,4 @@ const HomeStack = createStackNavigator(
   { DetailScreen, HomeScreen, OptionsScreen },
   { initialRouteName: 'HomeScreen', transitionConfig: dynamicModalTransition }
 );
-```
+```