Update hello_asset.adoc

Fixed image scaling.

src/docs/asciidoc/jme3/beginner/hello_asset.adoc

@@ -14,7 +14,7 @@ Next: <<jme3/beginner/hello_main_event_loop#,Hello Update Loop>>
 In this tutorial we will learn to load 3D models and text into the scene graph, using the jME <<jme3/advanced/asset_manager#,Asset Manager>>. You will also learn how to determine the correct paths, and which file formats to use.
 
 
-image::jme3/beginner/beginner-assets-models.png[beginner-assets-models.png,with="320",height="250",align="center"]
+image::jme3/beginner/beginner-assets-models.png[beginner-assets-models.png,320,250,align="center"]