Spring Change AsciiDoctor command with Antora in contributing.md

AsciiDoctor has been removed in april but the contributing.md still shows it as the command to build documentation. https://github.com/spring-projects/spring-framework/blob/main/CONTRIBUTING.md#reference-docs

Comment From: Nheyll

I opened a PR for this issue : #30739

Comment From: bclozel

Thanks! Closing in favor of #30739