Update asciidoctor-epub3 requirement from 2.1.0 to 2.1.3

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@v2.1.0...v2.1.3)

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

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

Author: dependabot[bot]

Gemfile

@@ -7,7 +7,7 @@ gem 'json', '2.7.2'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'
-gem 'asciidoctor-epub3', '2.1.0'
+gem 'asciidoctor-epub3', '2.1.3'
 gem 'asciidoctor-pdf', '2.3.15'
 
 gem 'coderay', '1.1.3'