Update asciidoctor-pdf requirement from 2.3.17 to 2.3.18

Updates the requirements on [asciidoctor-pdf](https://github.com/asciidoctor/asciidoctor-pdf) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-pdf/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-pdf/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-pdf@v2.3.17...v2.3.18)

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

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

Gemfile

@@ -8,7 +8,7 @@ gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'
 gem 'asciidoctor-epub3', '2.1.3'
-gem 'asciidoctor-pdf', '2.3.17'
+gem 'asciidoctor-pdf', '2.3.18'
 
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.4.1'