Bump asciidoctorj-pdf from 2.3.7 to 2.3.9 (#1072)

Co-authored-by: Boni Garcia <boni.garcia@uc3m.es>

pom.xml

@@ -32,7 +32,7 @@
         <exec-maven-plugin.version>3.1.0</exec-maven-plugin.version>
         <maven-jar-plugin.version>3.3.0</maven-jar-plugin.version>
         <maven-asciidoctor-plugin.version>2.2.4</maven-asciidoctor-plugin.version>
-        <asciidoctorj-pdf.version>2.3.7</asciidoctorj-pdf.version>
+        <asciidoctorj-pdf.version>2.3.9</asciidoctorj-pdf.version>
         <maven-site.version>3.12.1</maven-site.version>
         <maven-project-info-reports.version>3.4.5</maven-project-info-reports.version>
         <maven-javadoc-plugin.version>3.5.0</maven-javadoc-plugin.version>