fix(cloudformation): nested stack example in readme is broken (#5729)

Use `this` instead of `scope` and make `props` optional.

Fixes #5686

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/@aws-cdk/aws-cloudformation/README.md

@@ -68,19 +68,19 @@ import cfn = require('@aws-cdk/aws-cloudformation');
 import s3 = require('@aws-cdk/aws-s3');
 
 class MyNestedStack extends cfn.NestedStack {
-  constructor(scope: Construct, id: string, props: cfn.NestedStackProps) {
+  constructor(scope: Construct, id: string, props?: cfn.NestedStackProps) {
     super(scope, id, props);
 
     new s3.Bucket(this, 'NestedBucket');  
   }
 }
 
 class MyParentStack extends Stack {
-  constructor(scope: Construct, id: string, props: StackProps) {
+  constructor(scope: Construct, id: string, props?: StackProps) {
     super(scope, id, props);
 
-    new MyNestedStack(scope, 'Nested1');
-    new MyNestedStack(scope, 'Nested2');
+    new MyNestedStack(this, 'Nested1');
+    new MyNestedStack(this, 'Nested2');
   }
 }
 ```