Add missing screenOptions to Tab.Navigator (react-navigation#1127)

When I copied the code from "A native stack navigator for each tab", I realized the header is being duplicated. I checked that in the snack version, there is this "screenOptions" which is missing in the docs. I added it now.

Author: pooriamo

versioned_docs/version-6.x/tab-based-navigation.md

@@ -219,7 +219,7 @@ const Tab = createBottomTabNavigator();
 export default function App() {
   return (
     <NavigationContainer>
-      <Tab.Navigator>
+      <Tab.Navigator screenOptions={{ headerShown: false }}>
         <Tab.Screen name="Home" component={HomeStackScreen} />
         <Tab.Screen name="Settings" component={SettingsStackScreen} />
       </Tab.Navigator>