Fix download_models=True cli option

[mike-hearn]

The README contained the below cli option for downloading all models, but it was throwing an error.

# to download all model checkpoints (12 models in total)
python download_checkpoints.py --download_models=True

The error it was throwing was:

File "/code/prismer/download_checkpoints.py", line 51, in download_checkpoints
    assert all([m in _MODELS for m in download_models]), f"Invalid model name. Must be one of {_MODELS}"
TypeError: 'bool' object is not iterable

This PR should fix the error and download all the models when that flag is passed.

[lorenmt]

Thanks for the notice.