update readme

Author: fbchen

README.md

@@ -289,8 +289,6 @@ class ExampleState extends State<Example> {
 
 ```dart
 class ExampleState extends State<Example> {
-  ConstraintId topChild = ConstraintId();
-  ConstraintId bottomChild = ConstraintId();
   ConstraintId guideline = ConstraintId();
 
   @override
@@ -302,7 +300,6 @@ class ExampleState extends State<Example> {
             Container(
               color: const Color(0xFF005BBB),
             ).applyConstraint(
-              id: topChild,
               width: matchParent,
               height: matchConstraint,
               top: parent.top,
@@ -316,7 +313,6 @@ class ExampleState extends State<Example> {
             Container(
               color: const Color(0xFFFFD500),
             ).applyConstraint(
-              id: bottomChild,
               width: matchParent,
               height: matchConstraint,
               top: guideline.bottom,

lib/example.dart

@@ -9,8 +9,6 @@ class Example extends StatefulWidget {
 }
 
 class ExampleState extends State<Example> {
-  ConstraintId topChild = ConstraintId();
-  ConstraintId bottomChild = ConstraintId();
   ConstraintId guideline = ConstraintId();
 
   @override
@@ -22,7 +20,6 @@ class ExampleState extends State<Example> {
             Container(
               color: const Color(0xFF005BBB),
             ).applyConstraint(
-              id: topChild,
               width: matchParent,
               height: matchConstraint,
               top: parent.top,
@@ -36,7 +33,6 @@ class ExampleState extends State<Example> {
             Container(
               color: const Color(0xFFFFD500),
             ).applyConstraint(
-              id: bottomChild,
               width: matchParent,
               height: matchConstraint,
               top: guideline.bottom,