Fix error message in CrossFitEstimator (#58)

metalearners/cross_fit_estimator.py

@@ -184,8 +184,8 @@ def _predict_in_sample(
         if len(X) != sum(len(fold) for fold in self._test_indices):
             raise ValueError(
                 "Trying to predict in-sample on data that is unlike data encountered in training."
-                f"Training data included {len(self._test_indices)} observations while prediction "
-                f"data includes {len(X)} observations."
+                f"Training data included {sum(len(fold) for fold in self._test_indices)} "
+                f"observations while prediction data includes {len(X)} observations."
             )
         n_outputs = self._n_outputs(method)
         predictions = self._initialize_prediction_tensor(