Error in GS0 Developer Org

I am attempting to follow the steps here: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/openapi_beta.htm

And am using the URL as defined in Step 1 with Postman variables matching other endpoints in the Salesforce Collection: {{_endpoint}}/services/data/v{{version}}/async/specifications/oas3

And am using the request Body:

{
   "resources" : ["*"]
}

The response body tells me that there is an error in the request body"

[
    {
        "message": "Malformed request body, unable to identify 'resources' field",
        "errorCode": "REQUIRED_FIELD_MISSING",
        "fields": [
            "resources"
        ]
    }
]

Any suggestions?