Replace references to VisualServer in code comments with RenderingServer

VisualServer no longer exists in the `master` branch.

editor/editor_resource_preview.cpp

@@ -439,7 +439,7 @@ void EditorResourcePreview::stop() {
 		preview_sem.post();
 		while (!exited.is_set()) {
 			OS::get_singleton()->delay_usec(10000);
-			RenderingServer::get_singleton()->sync(); //sync pending stuff, as thread may be blocked on visual server
+			RenderingServer::get_singleton()->sync(); //sync pending stuff, as thread may be blocked on rendering server
 		}
 		thread.wait_to_finish();
 	}