DOC fix minor typo in TimeSeriesSplit docstrings (scikit-learn#31640)

Co-authored-by: Jérémie du Boisberranger <jeremie@probabl.ai>

Author: sylvaincom

sklearn/model_selection/_split.py

@@ -1110,10 +1110,10 @@ class TimeSeriesSplit(_BaseKFold):
     while the train set size accumulates data from previous splits.
 
     This cross-validation object is a variation of :class:`KFold`.
-    In the kth split, it returns first k folds as train set and the
-    (k+1)th fold as test set.
+    In the k-th split, it returns the first k folds as the train set and the
+    (k+1)-th fold as the test set.
 
-    Note that unlike standard cross-validation methods, successive
+    Note that, unlike standard cross-validation methods, successive
     training sets are supersets of those that come before them.
 
     Read more in the :ref:`User Guide <time_series_split>`.