Bump asciidoctorj-epub3 from 1.5.0-alpha.6 to 1.5.1 (#867)

Bumps [asciidoctorj-epub3](https://github.com/asciidoctor/asciidoctorj-epub3) from 1.5.0-alpha.6 to 1.5.1.
- [Release notes](https://github.com/asciidoctor/asciidoctorj-epub3/releases)
- [Commits](https://github.com/asciidoctor/asciidoctorj-epub3/commits/v1.5.1)

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

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

plugin/build.gradle

@@ -5,7 +5,7 @@ buildscript {
 	dependencies {
 		classpath "org.grails:grails-gradle-plugin:$grailsVersion"
 		classpath 'org.asciidoctor:asciidoctor-gradle-plugin:1.6.1'
-		classpath 'org.asciidoctor:asciidoctorj-epub3:1.5.0-alpha.6'
+		classpath 'org.asciidoctor:asciidoctorj-epub3:1.5.1'
 		classpath 'org.asciidoctor:asciidoctorj-pdf:1.5.4'
 	}
 }