Polish "Fix sample code on how to add plugin conditionally"

See gh-1068

initializr-docs/src/main/asciidoc/configuration-guide.adoc

@@ -82,12 +82,7 @@ to do something and makes the declaration more idiomatic. Consider the following
 
 [source,java,indent=0]
 ----
-	@Bean
+include::{code-examples}/doc/generator/project/ProjectCustomizationExamples.java[tag=war-plugin-contributor]
-	@ConditionalOnBuildSystem(GradleBuildSystem.ID)
-	@ConditionalOnPackaging(WarPackaging.ID)
-	public BuildCustomizer<GradleBuild> warPluginContributor() {
-		return (build) -> build.plugins().add("war");
-	}
 ----
 
 This registers a component that can customize a Gradle build only if the project to

initializr-docs/src/main/java/io/spring/initializr/doc/generator/project/ProjectCustomizationExamples.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.spring.initializr.doc.generator.project;
+
+import io.spring.initializr.generator.buildsystem.gradle.GradleBuild;
+import io.spring.initializr.generator.buildsystem.gradle.GradleBuildSystem;
+import io.spring.initializr.generator.condition.ConditionalOnBuildSystem;
+import io.spring.initializr.generator.condition.ConditionalOnPackaging;
+import io.spring.initializr.generator.packaging.war.WarPackaging;
+import io.spring.initializr.generator.spring.build.BuildCustomizer;
+
+import org.springframework.context.annotation.Bean;
+
+/**
+ * Examples of customizers.
+ *
+ * @author Stephane Nicoll
+ */
+public class ProjectCustomizationExamples {
+
+	// tag::war-plugin-contributor[]
+	@Bean
+	@ConditionalOnBuildSystem(GradleBuildSystem.ID)
+	@ConditionalOnPackaging(WarPackaging.ID)
+	public BuildCustomizer<GradleBuild> warPluginContributor() {
+		return (build) -> build.plugins().add("war");
+	}
+	// end::war-plugin-contributor[]
+
+}