Fix typos in the 0.9.0 document

Hi Danijar, I'm working with ninjax==0.9.0 since it is used in your DreamerV3 implementation. I found that your doc in https://pypi.org/project/ninjax/0.9.0/ has some typo error and will lead to the code fail to run.

In the "Quick Start" section, you have `assert state['/model/bias'].shape == ()`. But I found that the correct key is `assert state['model/bias'].shape == ()`. That's it.