docs(stepfunctions): fix typo in README (aws#10939)

The parameter specified for LogOptions is `destination`, not `destinations`.
https://github.com/aws/aws-cdk/blob/2640d9aa06039a6eb539d959d2126749486c7ef3/packages/%40aws-cdk/aws-stepfunctions/lib/state-machine.ts#L62

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

@@ -562,7 +562,7 @@ const logGroup = new logs.LogGroup(stack, 'MyLogGroup');
 new stepfunctions.StateMachine(stack, 'MyStateMachine', {
     definition: stepfunctions.Chain.start(new stepfunctions.Pass(stack, 'Pass')),
     logs: {
-      destinations: logGroup,
+      destination: logGroup,
       level: stepfunctions.LogLevel.ALL,
     }
 });