docs(python): Update GroupBy.__iter__ docstring to match new behavi…

…or (#17383)

py-polars/polars/dataframe/group_by.py

@@ -65,13 +65,12 @@ def __iter__(self) -> Self:
         Allows iteration over the groups of the group by operation.
 
         Each group is represented by a tuple of `(name, data)`. The group names are
-        tuples of the distinct group values that identify each group. If a single string
-        was passed to `by`, the keys are a single value instead of a tuple.
+        tuples of the distinct group values that identify each group.
 
         Examples
         --------
         >>> df = pl.DataFrame({"foo": ["a", "a", "b"], "bar": [1, 2, 3]})
-        >>> for name, data in df.group_by(["foo"]):  # doctest: +SKIP
+        >>> for name, data in df.group_by("foo"):  # doctest: +SKIP
         ...     print(name)
         ...     print(data)
         (a,)