fix missing imports

- Add the import for the data generator.
- Add the import for expit (sigmoid function)

docs/usage.rst

@@ -438,7 +438,9 @@ where
 To implement that as a DGP in JustCause we define the outcome function as follows::
 
  from sklearn.utils import check_random_state # ensures usable random state
-
+ from justcause.data.utils import generate_data
+ from scipy.special import expit
+ 
  def outcome(covariates, *, random_state: RandomState, **kwargs):
  random_state = check_random_state(random_state)