DOC: fix See also

Author: Axeldnahcram

pandas/core/groupby/groupby.py

@@ -2670,6 +2670,7 @@ def sem(self, ddof: int = 1, numeric_only: bool = False) -> NDFrameT:
         See Also
         --------
         DataFrame.sem : Return unbiased standard error of the mean over requested axis.
+        Series.sem : Return unbiased standard error of the mean over requested axis.
 
         Examples
         --------