Upgrade to spring-asciidoctor-backends 0.0.4

This release is available on Maven Central, whereas 0.0.3 is only
present on repo.spring.io/release.

spring-graphql-docs/build.gradle

@@ -25,7 +25,7 @@ dependencies {
 	api 'org.springframework:spring-messaging'
 	api 'org.springframework.data:spring-data-commons'
 	api 'com.querydsl:querydsl-core'
-	asciidoctorExtensions 'io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.3'
+	asciidoctorExtensions 'io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.4'
 }
 
 jar {
@@ -36,16 +36,6 @@ javadoc {
 	enabled = false
 }
 
-
-repositories {
-	maven {
-		url "https://repo.spring.io/release"
-		mavenContent {
-			includeGroup "io.spring.asciidoctor.backends"
-		}
-	}
-}
-
 ext.javadocLinks = [
 		"https://docs.oracle.com/javase/8/docs/api/",
 		"https://javadoc.io/doc/com.graphql-java/graphql-java/${graphQlJavaVersion}/",