chore(aws-eks): update README with correct service account syntax (#8112

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chore(aws-eks): update README with correct service account syntax

`serviceAccountName` should be at `spec.serviceAccountName` and add a CfnOutput sample to demo how to get the IAM role of this service account.

My working sample here
https://twitter.com/pahudnet/status/1263286407092514817



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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

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

@@ -526,26 +526,33 @@ With services account you can provide Kubernetes Pods access to AWS resources.
 
 ```ts
 // add service account
-const serviceAccount = cluster.addServiceAccount('MyServiceAccount');
+const sa = cluster.addServiceAccount('MyServiceAccount');
 
 const bucket = new Bucket(this, 'Bucket');
 bucket.grantReadWrite(serviceAccount);
 
-cluster.addResource('mypod', {
+const mypod = cluster.addResource('mypod', {
   apiVersion: 'v1',
   kind: 'Pod',
   metadata: { name: 'mypod' },
   spec: {
+    serviceAccountName: sa.serviceAccountName
     containers: [
       {
         name: 'hello',
         image: 'paulbouwer/hello-kubernetes:1.5',
         ports: [ { containerPort: 8080 } ],
-        serviceAccountName: serviceAccount.serviceAccountName
+
       }
     ]
   }
 });
+
+// create the resource after the service account
+mypod.node.addDependency(sa);
+
+// print the IAM role arn for this service account
+new cdk.CfnOutput(this, 'ServiceAccountIamRole', { value: sa.role.roleArn })
 ```
 
 ### Roadmap