Update openapi.md

[minnieshi]

remove obsolte method, add maven part into the existing method

the is archived, which indicates it is not actively maintained/used https://github.com/ghostbuster91/sttp-openapi-generator

Before submitting pull request:

• Check if the project compiles by running sbt compile
• Verify docs compilation by running sbt compileDocs
• Check if tests pass by running sbt test
• Format code by running sbt scalafmt

[adamw]

Thanks!