Update SpringBootServletInitializer ref docs

Add a hint to show that the `configure` method is now optional.

Fixes gh-2074

Author: philwebb

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2092,7 +2092,10 @@ Example:
 
 		@Override
 		protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
-			return application.sources(Application.class);
+			// Customize the application or call application.sources(...) to add sources
+			// Since our example is itself a @Configuration class we actually don't
+			// need to override this method.
+			return application;
 		}
 
 	}