DOC PR09 Add missing . on freq parameter on groupby.py (#31998)

Author: za

pandas/core/groupby/groupby.py

@@ -2329,7 +2329,7 @@ def shift(self, periods=1, freq=None, axis=0, fill_value=None):
         periods : int, default 1
             Number of periods to shift.
         freq : str, optional
-            Frequency string
+            Frequency string.
         axis : axis to shift, default 0
         fill_value : optional