Fixed bug in "Adding Uncertainty over Model Parameters" section of the tutorial

[michaelleejl]

In Adding Uncertainty over Model Parameters, (https://basisresearch.github.io/chirho/tutorial_i.html#Adding-Uncertainty-over-Model-Parameters), the following code snippet

def bayesian_population_causal_model(n_individuals):
    stress_prob, smokes_cond_prob, cancer_cond_prob = parameter_prior()
    return population_intervened_causal_model(
        n_individuals, stress_prob, smokes_cond_prob, cancer_cond_prob
    )

Calls population_intervened_causal_model. It should instead call population_causal_model.