Fix Franka kitchen XML load error

[Taokt]

We have tried to run "python examples/train.py" but encountered:

Initializing Franka sim
Error executing job with overrides: []
Error in call to target 'gym.envs.registration.make':
ValueError("XML Error: top-level default class 'main' cannot be renamed\nElement 'default', line 0\n")
full_key: env.gym.env

After (a lot of) searching, we found out the solution is Farama-Foundation/Gymnasium-Robotics#255

Thought it could be helpful for people who are facing the same problem.