Update link to AsciiDoc quick reference in readme

[HonkingGoose]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Update link to AsciiDoc quick reference

Context

The old link redirects to the new docs.asciidoctor.org domain.
I think it makes sense to fix our link, so we're not relying on them redirecting old link users.