Make Rolling.apply documentation clearer (#25712)

Author: bharatr21

pandas/core/window.py

@@ -953,7 +953,7 @@ def count(self):
     ----------
     func : function
         Must produce a single value from an ndarray input if ``raw=True``
-        or a Series if ``raw=False``.
+        or a single value from a Series if ``raw=False``.
     raw : bool, default None
         * ``False`` : passes each row or column as a Series to the
           function.