Update asciidoctor-epub3 requirement from 1.5.1 to 2.0.0 (#37)

Updates the requirements on [asciidoctor-epub3](https://github.com/asciidoctor/asciidoctor-epub3) to permit the latest version.
- [Release notes](https://github.com/asciidoctor/asciidoctor-epub3/releases)
- [Changelog](https://github.com/asciidoctor/asciidoctor-epub3/blob/main/CHANGELOG.adoc)
- [Commits](asciidoctor/asciidoctor-epub3@v1.5.1...v2.0.0)

---
updated-dependencies:
- dependency-name: asciidoctor-epub3
  dependency-type: direct:production
...

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

Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.7.1'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'
-gem 'asciidoctor-epub3', '1.5.1'
+gem 'asciidoctor-epub3', '2.0.0'
 gem 'asciidoctor-pdf', '2.3.10'
 
 gem 'coderay', '1.1.3'