Polish "Fix syntax errors in docs"

Closes gh-17835

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -7933,26 +7933,10 @@ for assertions, as follows:
 
 [source,java,indent=0]
 ----
-	import org.junit.Rule;
+include::{test-examples}/test/rule/OutputCaptureTests.java[tag=test]
-	import org.junit.Test;
+----
-	import org.springframework.boot.test.rule.OutputCapture;
-
-	import static org.hamcrest.Matchers.*;
-	import static org.junit.Assert.*;
-
-	public class MyTest {
-
-		@Rule
-		public OutputCapture capture = new OutputCapture();
 
-		@Test
-		public void testName() throws Exception {
-			System.out.println("Hello World!");
-			assertThat(capture.toString(), containsString("World");
-		}
 
-	}
-----
 
 [[boot-features-rest-templates-test-utility]]
 ==== TestRestTemplate

spring-boot-project/spring-boot-docs/src/test/java/org/springframework/boot/docs/test/rule/OutputCaptureTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2012-2019 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 org.springframework.boot.docs.test.rule;
+
+import org.junit.Rule;
+import org.junit.Test;
+
+import org.springframework.boot.test.rule.OutputCapture;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Sample showcasing the use of {@link OutputCapture}.
+ *
+ * @author Stephane Nicoll
+ */
+// tag::test[]
+public class OutputCaptureTests {
+
+	@Rule
+	public final OutputCapture capture = new OutputCapture();
+
+	@Test
+	public void testName() {
+		System.out.println("Hello World!");
+		assertThat(this.capture.toString()).contains("World");
+	}
+
+}
+// end::test[]