docs: updated app delivery instructions for self update pipeline

[robbytaylor]

I was trying to follow the example in the app delivery README to create a self updating CodeDeploy pipeline but I think some of the APIs have been changed since the example was written as some method calls were no longer valid.

I've tried to update the example as best as I could to make it functional, although to some extent I didn't completely understand what the example was intended to do (that's why I was following an example!) so I may have missed the point somewhat. I'm happy to adapt this as required if I've missed out something that was important.

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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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.