Fix call to pm.sample in docs

[henrytdsimmons]

Using the docs example:

with model:
    trace = pm.sample(
        model,
        nuts_sampler="nutpie",
        nuts_sampler_kwargs={"backend": "jax"},
    )

Results in:
TypeError: '<' not supported between instances of 'Model' and 'int'

This is because model is not the first argument for pm.sample when in the with block.

Removed model positional argument from both examples.

[aseyboldt]

Thank you!