Update asciidoctor-pdf requirement from 1.6.0 to 1.6.1 (progit#260)

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@v1.6.0...v1.6.1)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.5.1'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-epub3', '1.5.1'
-gem 'asciidoctor-pdf', '1.6.0'
+gem 'asciidoctor-pdf', '1.6.1'
 
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.2.0'