Update asciidoctor-pdf requirement from 2.0.0 to 2.1.1 (progit#282)

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@v2.0.0...v2.1.1)

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

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Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.6.2'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-epub3', '1.5.1'
-gem 'asciidoctor-pdf', '2.0.0'
+gem 'asciidoctor-pdf', '2.1.1'
 
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.3.0'