Fix load_params error reporting expected instead of actual checkpoint type

[PratikDhanave]

In gemma/gm/ckpts/_checkpoint.py, load_params validates the caller-provided params:

if params.type != _CheckpointType.NESTED:
    raise ValueError(
        'The input params provided to `load_params()` should be the raw'
        " model params matching the Flax `model.init()['params']` structure."
        f' Got: {_CheckpointType.NESTED}'
    )

The Got: suffix interpolates the constant _CheckpointType.NESTED — the expected value — so the message always reads Got: ...NESTED even though we only reach this branch when params.type is something else. Report params.type (the actual value that failed the check) instead, so the error tells the user what they actually passed.