Fix docs references to html files (use .adoc)

[francoisfreitag]

When browsing the documentation on GitHub, the links are broken as they
refer to an HTML document, which has been changed to adoc in commit
ff4f5b4.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[MarkEWaite]

Thanks for fixing the broken link. I've confirmed that link is broken on the master branch and that this change fixes it.

[francoisfreitag]

The parameter injection link at https://github.com/jenkinsci/stapler/blob/master/docs/reference.adoc#action-method is broken, but I didn’t see a replacement.

[basil]

The parameter injection link at https://github.com/jenkinsci/stapler/blob/master/docs/reference.adoc#action-method is broken, but I didn’t see a replacement.

According to this page, this content once lived at http://stapler.java.net/parameter-injection.html, but that page is not even present on archive.org. Unless @kohsuke has a backup, I think this content is lost to the mists of time.