Fix broken links to extension api documentation

[djarnis73]

Thank you for opening a pull request and contributing to asciidoctor-maven-plugin!

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Documentation
• Refactor
• Build improvement
• Other (please describe)

What is the goal of this pull request?
Improve documentation

Is it related to an existing issue?

• Yes
• No

Finally, please add a corresponding entry to CHANGELOG.adoc
I don't think this warrants a changelog entry.

[coveralls]

Coverage remained the same at 81.827% when pulling 756d076 on djarnis73:fix-broken-links-to-extensions-api into 329c13a on asciidoctor:master.

[abelsromero]

Thanks a lot!
Do you care updating the CHANGELOG too? You'll have to add Documentation section for the current Unreleased.

[djarnis73]

Thanks a lot!
Do you care updating the CHANGELOG too? You'll have to add Documentation section for the current Unreleased.

Personally I think this change is so insignificant that it does not warrant en entry in the changelog, but I will add one if you prefer.

Best regards Jens

[abelsromero]

Personally I think this change is so insignificant that it does not warrant en entry in the changelog, but I will add one if you prefer.

Something was not working and now it is, I think is relevant enough.