Fix JsonSerializer example in reference guide

See gh-30329

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/developingwebapplications/springmvc/json/MyJsonComponent.java

@@ -37,8 +37,10 @@ public static class Serializer extends JsonSerializer<MyObject> {
 
 		@Override
 		public void serialize(MyObject value, JsonGenerator jgen, SerializerProvider serializers) throws IOException {
+			jgen.writeStartObject();
 			jgen.writeStringField("name", value.getName());
 			jgen.writeNumberField("age", value.getAge());
+			jgen.writeEndObject();
 		}
 
 	}