Revert "Add "required" to example for null-safety (flutter#5416)" (fl…

…utter#5479)

This reverts commit 3238fa2.

src/docs/development/ui/widgets-intro.md

@@ -98,7 +98,7 @@ Below are some simple widgets that combine these and other widgets:
 import 'package:flutter/material.dart';
 
 class MyAppBar extends StatelessWidget {
-  MyAppBar({required this.title});
+  MyAppBar({this.title});
 
   // Fields in a Widget subclass are always marked "final".