fix xref

docs/modules/tutorials/pages/beginner/hello_asset.adoc

@@ -250,7 +250,7 @@ java.lang.NullPointerException
 
 You see that loading the *XML/OBJ/GLTF* directly is only acceptable during the development phase in the SDK. For example, every time your graphic designer pushes updated files to the asset directory, you can quickly review the latest version in your development environment.
 
-But for QA test builds and for the final release build, you use *.j3o files* exclusively. J3o is an optimized binary format for jME3 applications. When you do QA test builds, or are ready to release, use the xref:ROOT:sdk.adoc[SDK] to xref:sdk:model_loader_and_viewer.adoc[convert] all .obj/.scene/.xml/.gltf files to .j3o files, and update all code to load the .j3o files. The default build script automatically packages .j3o files in the executables.
+But for QA test builds and for the final release build, you use *.j3o files* exclusively. J3o is an optimized binary format for jME3 applications. When you do QA test builds, or are ready to release, use the xref:sdk:sdk.adoc[SDK] to xref:sdk:model_loader_and_viewer.adoc[convert] all .obj/.scene/.xml/.gltf files to .j3o files, and update all code to load the .j3o files. The default build script automatically packages .j3o files in the executables.
 
 Open your JME3 Project in the jMonkeyEngine SDK.