docs(apigateway): correct TokenAuthorizer example (aws#5619)

The README incorrectly used `lambda.TokenAuthorizer` in the Lambda-based
token authorizer example

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

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

@@ -381,7 +381,7 @@ const authFn = new lambda.Function(this, 'booksAuthorizerLambda', {
   // ...
 });
 
-const auth = new lambda.TokenAuthorizer(this, 'booksAuthorizer', {
+const auth = new apigateway.TokenAuthorizer(this, 'booksAuthorizer', {
   function: authFn
 });