Fix warning print when running asset shelf UI template

Was printing the following warning:
  Warning: 'MyAssetShelf' does not contain '_AST_' with prefix and suffix

I followed some other template for the naming, apparently the "Ui Tool
Simple" one, but that doesn't print the warning.

scripts/templates_py/ui_asset_shelf.py

@@ -3,7 +3,7 @@ import bpy
 
 class MyAssetShelf(bpy.types.AssetShelf):
     bl_space_type = 'VIEW_3D'
-    bl_idname = "my_template.my_material_asset_shelf"
+    bl_idname = "VIEW3D_AST_my_asset_shelf"
 
     @classmethod
     def poll(cls, context):