Bump org.asciidoctor:asciidoctorj-epub3 from 2.0.1 to 2.1.0 (#257)

Bumps [org.asciidoctor:asciidoctorj-epub3](https://github.com/asciidoctor/asciidoctorj-epub3) from 2.0.1 to 2.1.0.
- [Commits](asciidoctor/asciidoctorj-epub3@v2.0.1...v2.1.0)

---
updated-dependencies:
- dependency-name: org.asciidoctor:asciidoctorj-epub3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

asciidoctor-epub-example/pom.xml

@@ -15,7 +15,7 @@
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <asciidoctor.maven.plugin.version>3.0.0</asciidoctor.maven.plugin.version>
-        <asciidoctorj.epub.version>2.0.1</asciidoctorj.epub.version>
+        <asciidoctorj.epub.version>2.1.0</asciidoctorj.epub.version>
         <asciidoctorj.version>2.5.11</asciidoctorj.version>
         <!-- for a correct kf8 generation, you'll need at least jRuby >= 9.1.8.0 -->
         <jruby.version>9.4.5.0</jruby.version>