[MINOR][PYTHON][DOCS] Fix the doctest of pivot

[zhengruifeng]

What changes were proposed in this pull request?

Why are the changes needed?

Fix the doctest of pivot, to make sure the example works

Does this PR introduce any user-facing change?

doc-only change

How was this patch tested?

enabled doc-test

Was this patch authored or co-authored using generative AI tooling?

no

[dongjoon-hyun]

Oh, could you check the pivot test failure?

File "/__w/spark/spark/python/pyspark/sql/connect/group.py", line 309, in pyspark.sql.connect.group.GroupedData.pivot
Failed example:
    df2.groupBy("sales.year").pivot(
        "sales.course"
    ).sum("sales.earnings").sort("year").show()
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.11/doctest.py", line 1355, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest pyspark.sql.connect.group.GroupedData.pivot[7]>", line 3, in <module>
        ).sum("sales.earnings").sort("year").show()
          ^^^^^^^^^^^^^^^^^^^^^
      File "/__w/spark/spark/python/pyspark/sql/connect/group.py", line 208, in sum
        return self._numeric_agg("sum", list(cols))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/__w/spark/spark/python/pyspark/sql/connect/group.py", line 175, in _numeric_agg
        raise PySparkTypeError(
    pyspark.errors.exceptions.base.PySparkTypeError: [NOT_NUMERIC_COLUMNS] Numeric aggregation function can only be applied on numeric columns, got ['sales.earnings'].
**********************************************************************
   1 of   8 in pyspark.sql.connect.group.GroupedData.pivot
***Test Failed*** 1 failures.

[dongjoon-hyun]

+1, LGTM (Pending CIs).

[dongjoon-hyun]

Merged to master. Thank you.