Update src/sim/plugins/hello_world.cc

src/sim/plugins/hello_world.cc

+#include <gazebo/gazebo.hh>
+
+namespace gazebo
+{
+  class WorldPluginTutorial : public WorldPlugin
+  {
+    public: WorldPluginTutorial() : WorldPlugin()
+            {
+              printf("Hello World!\n");
+            }
+
+    public: void Load(physics::WorldPtr _world, sdf::ElementPtr _sdf)
+            {
+            }
+  };
+  GZ_REGISTER_WORLD_PLUGIN(WorldPluginTutorial)
+}