Fixes docstring for UsdFileCfg (#1554)

# Description

Fixes docstring for UsdFileCfg
Signed-off-by: Kelly Guo <kellyg@nvidia.com>

source/extensions/omni.isaac.lab/omni/isaac/lab/sim/spawners/from_files/from_files_cfg.py

@@ -72,8 +72,7 @@ class UsdFileCfg(FileCfg):
     ones. These include:
 
     - **Selecting variants**: This is done by specifying the :attr:`variants` parameter.
-    - **Creating and applying materials**: This is done by specifying the :attr:`visual_material` and
-      :attr:`physics_material` parameters.
+    - **Creating and applying materials**: This is done by specifying the :attr:`visual_material` parameter.
     - **Modifying existing properties**: This is done by specifying the respective properties in the configuration
       class. For instance, to modify the scale of the imported prim, set the :attr:`scale` parameter.