fix: update asciidoctor requirement from 2.0.16 to 2.0.18

Updates the requirements on [asciidoctor](https://github.com/asciidoctor/asciidoctor) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor@v2.0.16...v2.0.18)

---
updated-dependencies:
- dependency-name: asciidoctor
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -2,7 +2,7 @@ source 'https://rubygems.org'
 
 gem "github-markup", "4.0.1"
 gem "RedCloth", "=4.3.2"
-gem "asciidoctor", "2.0.16"
+gem "asciidoctor", "2.0.18"
 gem "commonmarker", "=0.21.0"
 gem "creole", "=0.5.0"
 gem "org-ruby", "=0.9.12"