Fix response json example for http 200 on entity deletion

[spwoodcock]

What is included in this PR?

• For the Deleting an Entity section in the Central API docs, the response example for a HTTP 200 is incorrect.
• The docs state that it is a JSON response, with a string value {'message': 'success'}:

• In fact, on testing, the actual response is a JSON with bool value {'success': True}.
• I updated the docs to reflect this.

Context

• I wasn't sure if the OpenAPI docs are auto-generated somehow, but I made a PR anyway to update the yaml manually.
• Feel free to close this PR and update the upstream if that is the case.

[lognaturel]

Thanks! Indeed the API docs are managed in the Central backend repo. I've made that clearer in this readme and made the fix at getodk/central-backend#1121