[BUG] Documentation problem on how to running the tests when editing a template

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

https://github.com/openapitools/openapi-generator/blob/master/CONTRIBUTING.md

Say's "During development, it can be helpful to quickly regenerate the samples without recompiling all of openapi-generator, e.g. when you have only updated the mustache templates. This can be done by passing the -t parameter: -t modules/openapi-generator/src/main/resources/python."

What program is that "-t" an argument to?

Please give an example in the context of python like the surrponding text.

Also when following the documentation to the best of my ability the test for python-flask (the one I'm editing) fail.
Because the instructions don't explain anywhere that to make the tests pass then a server has to be running, not how to run the server.

Need better instructions for plain old dev.

Start with CONTRIBUTING.md and locate the template regeneration instructions containing the -t option. Reproduce the documented python-flask test workflow, determine what server setup the issue refers to, and update the documentation with a Python example, the relevant command context, and server-running instructions; done when a developer can follow the instructions and run the test successfully.