update docstring for runnin with older pyarrow (i.e. using Python backend)

Author: jorisvandenbossche

pandas/core/strings/accessor.py

@@ -3340,7 +3340,7 @@ def casefold(self):
     >>> s3.str.isdigit()
     0     True
     1     True
-    2     True
+    2    False
     3    False
     dtype: bool