update type notation

Author: qroa

mapie/regression/quantile_regression.py

@@ -600,7 +600,7 @@ def fit_estimators(
         shuffle: Optional[bool] = True,
         stratify: Optional[ArrayLike] = None,
         **fit_params,
-    ):
+    ) -> Tuple[ArrayLike, ArrayLike]:
 
         self._check_parameters()
         checked_estimator = self._check_estimator(self.estimator)