biggym jump__cheetah doesn't work

Easiest way to demonstrate it is to go to https://colab.research.google.com/github/google/brax/blob/main/notebooks/biggym/biggym_rl.ipynb and do these two changes: `env_name = 'jump__cheetah'` and `env_params = {}`
This will throw an error:
```
---------------------------------------------------------------------------

TypeError Traceback (most recent call last)

[](https://localhost:8080/#) in ()
11 env_fn = composer.create_fn(env_name=env_name, **env_params)
12 # env_fn = biggym.create_fn(env_name=env_name, **env_params)
---> 13 env = env_fn()
14 show_env(env)

[/usr/local/lib/python3.7/dist-packages/brax/experimental/composer/composer.py](https://localhost:8080/#) in create(env_name, env_desc, desc_edits, episode_length, action_repeat, auto_reset, batch_size, **kwargs)
477 else:
478 assert not kwargs, f'unused kwargs: {kwargs}'
--> 479 env_desc = dict(**env_desc, **desc)
480 env_desc = composer_utils.edit_desc(env_desc, desc_edits)
481 composer = Composer(**env_desc)

TypeError: type object argument after ** must be a mapping, not JumpCheetah
```

All other biggym envs have mappings as `ENV_DESCS` and work fine, while jump__cheetah points to a class: