Use ServiceLoader class to obtain service instance

In the "main-app" module, the `HelloInterface` service instance, `HelloModules`, was just being treated like an ordinary class that was exported from the "hello.modules" module. The code was not treating the class as a service class. This is not a good example of how services are used.

This commit makes use of the `ServiceLoader` class, which provides access to the `HelloInterface` instance as defined in the "hello.modules" module. This serves as a better example for how services are used.

core-java-modules/core-java-9-jigsaw/src/simple-modules/main.app/com/baeldung/modules/main/MainApp.java

@@ -1,12 +1,15 @@
 package com.baeldung.modules.main;
 
+import com.baeldung.modules.hello.HelloInterface;
 import com.baeldung.modules.hello.HelloModules;
+import java.util.ServiceLoader;
 
 public class MainApp {
     public static void main(String[] args) {
         HelloModules.doSomething();
-
-        HelloModules module = new HelloModules();
-        module.sayHello();
+        
+        Iterable<HelloInterface> services = ServiceLoader.load(HelloInterface.class);
+        HelloInterface service = services.iterator().next();
+        service.sayHello();
     }
 }