docs(codepipeline-actions): update CloudTrail example for addS3EventS…

…elector (#13728)

Fixes #13684


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packages/@aws-cdk/aws-codepipeline-actions/README.md

@@ -232,7 +232,10 @@ import * as cloudtrail from '@aws-cdk/aws-cloudtrail';
 
 const key = 'some/key.zip';
 const trail = new cloudtrail.Trail(this, 'CloudTrail');
-trail.addS3EventSelector([sourceBucket.arnForObjects(key)], {
+trail.addS3EventSelector([{
+  bucket: sourceBucket,
+  objectPrefix: key,
+}], {
   readWriteType: cloudtrail.ReadWriteType.WRITE_ONLY,
 });
 const sourceAction = new codepipeline_actions.S3SourceAction({