Added CreatePrivateZone example ref to swagger spec

[markjbrown]

This checklist is used to make sure that common issues in a pull request are addressed. This will expedite the process of getting your pull request merged and avoid extra work on your part to fix issues discovered during the review process.

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[sergey-shandar]

@lmazuel It looks like, the correct way to extend a model is to add "properties" inside "allOf" https://spacetelescope.github.io/understanding-json-schema/structuring.html#extending It's possible that AutoRest is smart enough to handle a case when "properties" are outside of "allOf".

[lmazuel]

After discussing with @sergey-shandar , merging with an OAV bug:
Azure/oav#280

Doesn't mean the Swagger is correct, but could be fixed later if the OAV bug is invalid.