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

[Josh00-Lu]

Description

There's a missing 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 (if needed).

Related issues $ temp fixes includes:
Farama-Foundation/D4RL#236 (comment)
CleanDiffuserTeam/CleanDiffuser#24
Farama-Foundation/D4RL#238 (comment)

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Notes

This behavior aligns with other assets descriptions in kitchen. For example, in microwave:

    <default>
        <default class="microwave">
            <joint damping="2" frictionloss="2" armature=".01" limited="true"/>
            <geom conaffinity="0" contype="0" group="1" material="micro_black" type="mesh"/>
            <default class="micro_collision">
                <geom conaffinity="1" condim="3" contype="0" group="4" margin="0.001" material="micro_collision_blue" solimp=".8 .9 .01" solref=".02 1"/>
            </default>
        </default>
    </default>

in oven:

    <default>
        <default class="oven">
            <joint armature="0.001" damping="2" limited="true"/>
            <geom conaffinity="0" contype="0" group="1" material="oven_metal" type="mesh"/>
            <default class="ovenlight" >
                <light directional="false" castshadow="true" attenuation="0.03 0.03 0.03" cutoff="100" exponent="25" diffuse=".7 .65 .65" specular=".3 .3 .3"/>
            </default>
            <default class="oven_collision">
                <geom conaffinity="1" condim="3" contype="0" group="4" margin="0.001" material="oven_collision_blue" type="mesh"/>
            </default>
        </default>
    </default>

However, in kettle

    <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>

I hope this typo could be fixed soon. Since in latest version of MuJoCo, the default tag without name is identified as "main default", which will bring errors like:

ValueError: XML Error: top-level default class 'main' cannot be renamed

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Kallinteris-Andreas]

Please also validate fot version 2 of MuJoCo, and provide the code you used to validate it.