Update DrawerNavigator.md (#212)

Button requires a title prop, and its same in other pages(tabbarNavigator, stackbarNavigator) but here it is label.

packages/react-navigation/docs/api/navigators/DrawerNavigator.md

@@ -20,7 +20,7 @@ class MyHomeScreen extends React.Component {
     return (
       <Button
         onPress={() => this.props.navigation.navigate('Notifications')}
-        label="Go to notifications"
+        title="Go to notifications"
       />
     );
   }
@@ -43,7 +43,7 @@ class MyNotificationsScreen extends React.Component {
     return (
       <Button
         onPress={() => this.props.navigation.goBack()}
-        label="Go back home"
+        title="Go back home"
       />
     );
   }