Fixed example to use new parameter name

Fixed example to align with changes from [this commit](aws#2857)

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

@@ -41,7 +41,7 @@ const submitJob = new sfn.Task(this, 'Submit Job', {
 });
 
 const waitX = new sfn.Wait(this, 'Wait X Seconds', {
-    duration: sfn.WaitTime.secondsPath('$.waitSeconds'),
+    time: sfn.WaitTime.secondsPath('$.waitSeconds'),
 });
 
 const getStatus = new sfn.Task(this, 'Get Job Status', {