Fix asciidoctor-maven-plugin's phase

[making]

The document (6) explains prepare-package should be used, but the example is package.

Actually, I had a trouble copying and pasting according to this doc.

[wilkinsona]

Sorry that you found this confusing. The need to use prepare-package if you want to include the documentation in the jar file is mentioned in the call out referenced by (6):

(6) If you want to package the documentation in your project’s jar you should use the prepare-package phase.

It's also mentioned again in the Packaging the documentation section:

First, configure the Asciidoctor plugin so that it runs in the prepare-package phase, as described above

If you have suggestions for how to make this clearer, I'd definitely like to hear them. However, I can't merge this change in its current form as it'll mean that the sections mentioned above (among others) no longer make sense.

[making]

Thank you for your comment.
I found I had misunderstood.

Updated PR to explain the phase explicitly.
How do you think?

[wilkinsona]

Having thought about this some more, I think it might be best to update the documentation to always use prepare-package for the phase. It'll be a lot simpler as it'll mean one set of instructions that'll work for those who want to embed the documentation in the jar and those that don't.

[wilkinsona]

Thanks for the PR. As discussed above, I've decided to address the problem by recommending that the Asciidoctor plugin is always bound to the prepare-package phase. See #218 for that change.