Remove duplicate eruby option in asciidoctor task

docs/guides/spring-security-docs-guides.gradle

@@ -3,7 +3,6 @@ apply plugin: 'org.asciidoctor.gradle.asciidoctor'
 asciidoctor {
 	baseDir = file('src/docs/asciidoc')
 	options = [
-	  eruby: 'erubis',
 	  eruby: 'erubis',
 	  attributes: [
 		  copycss : '',
@@ -43,4 +42,4 @@ def getMavenRepositoryInclude() {
 	} else {
 		return "_includes/maven-repository-milestone.asc"
 	}
-}
+}