Fix bugs in kettle_asset.xml to ensure compatibility with the latest …

…MuJoCo versions

There's a missing <default> tag in kettle_asset.xml. This will bring errors in the latest version of MuJoCo (>3.16). I have validated that this change does not change the behavior of the environment for older versions of mujoco (3.1.6, 3.0.1). However, the behavior in MuJoCo version (<3.0, e.g. 2.1.5) remains to be checked.

gymnasium_robotics/envs/assets/kitchen_franka/kitchen_assets/item_assets/kettle_asset.xml

@@ -13,11 +13,13 @@
         <material name="kettle_white" rgba=".9 .9 .9 1" reflectance="1" shininess="1" />
         <material name="kettle_collision_blue" rgba="0.3 0.3 1.0 0.5" shininess="0" specular="0" />
     </asset>
-    <default class="kettle">
-        <joint damping="2" frictionloss="2" armature=".01" limited="true" />
-        <geom conaffinity="0" contype="0" group="1" material="kettle_white" type="mesh"/>
-        <default class="kettle_collision">
-            <geom conaffinity="1" condim="4" contype="1" group="4" margin="0.001" material="kettle_collision_blue" solimp=".8 .9 .01" solref=".02 1" type="mesh"/>
+    <default>
+        <default class="kettle">
+            <joint damping="2" frictionloss="2" armature=".01" limited="true" />
+            <geom conaffinity="0" contype="0" group="1" material="kettle_white" type="mesh"/>
+            <default class="kettle_collision">
+                <geom conaffinity="1" condim="4" contype="1" group="4" margin="0.001" material="kettle_collision_blue" solimp=".8 .9 .01" solref=".02 1" type="mesh"/>
+            </default>
         </default>
     </default>
 </mujocoinclude>