Clarify options passed in simple Petstore example.

Description

I recently had a consumer of my API reach out and ask, "The Swagger docs indicate that we’ll need a swagger.json file to build the library. I haven’t been able to find a specific one for your api, just the example on the Swagger repo. So unless that’s forthcoming with the API creds, I might need to be pointed in the right direction." My team uses YAML for our specification file. This question ended up taking a bit of back and forth to discover what the consumer meant because we don't have a 'swagger.json' file we have a YAML file.

My proposal is to add some comments to the generate command for the pet store example to describe the different options that are being passed. The goal being to call out that the http://petstore.swagger.io/v2/swagger.json file is the specification file so that if a situation similar to mine happens again the consumer would ask where the specification file was instead of asking where the swagger.json file can be found.

Swagger-codegen version

3.X

Swagger declaration file content or url

N/A

Command line used for generation

N/A

Steps to reproduce

N/A

Related issues/PRs

Suggest a fix/enhancement

I created a PR for my suggested change to the README.md. https://github.com/swagger-api/swagger-codegen/pull/10506

Start with README.md and inspect the simple Petstore example's generate command. Compare its options with the existing explanation, especially the specification-file argument. Done means the README clearly explains that the Petstore URL is the specification file; a related pull request is already linked.