Clarify documentation about specifying main class

See gh-17003

spring-boot-project/spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -116,10 +116,21 @@ the usual `packaging` element, as shown in the following example:
 ----
 
 Your existing archive is enhanced by Spring Boot during the `package` phase. The main
-class that you want to launch can be specified either by using a configuration option or
-by adding a `Main-Class` attribute to the manifest in the usual way. If you do not specify
-a main class, the plugin searches for a class with a
-`public static void main(String[] args)` method.
+class that you want to launch can be specified either by using a configuration option, as
+shown below, or by adding a `Main-Class` attribute to the manifest. If you do not specify
+a main class, the plugin searches for a class with a `public static void main(String[] args)`
+method.
+[source,xml,indent=0,subs="verbatim,attributes"]
+----
+	<plugin>
+		<groupId>org.springframework.boot</groupId>
+		<artifactId>spring-boot-maven-plugin</artifactId>
+		<configuration>
+			<mainClass>com.example.app.Main</mainClass>
+		</configuration>
+	</plugin>
+----
+
 
 To build and run a project artifact, you can type the following: