humanoid_run doesn't work in composer Colab

If you go to:
https://colab.research.google.com/github/google/brax/blob/main/notebooks/composer/composer.ipynb
and choose `humanoid_run` as the environment, the next cell throws the following error:
```
[/usr/local/lib/python3.7/dist-packages/brax/physics/system.py](https://localhost:8080/#) in default_angle(self, default_index)
68 defaults = self.config.defaults[default_index]
69 for ja in defaults.angles:
---> 70 arr = vec_to_arr(ja.angle)[:dofs[ja.name]] * jp.pi / 180
71 angles[ja.name] = arr
72

KeyError: 'left_knee'
```