Fix PDF docs

Resolves: #1603

Author: eleftherias

spring-session-docs/spring-session-docs.gradle

@@ -25,7 +25,7 @@ dependencies {
 
 def versions = dependencyManagement.managedVersions
 
-asciidoctor {
+asciidoctorj {
 	def ghTag = snapshotBuild ? 'master' : project.version
 	def ghUrl = "https://github.com/spring-projects/spring-session/tree/$ghTag"