docs: include the clean-up step in the udf code snippet

[shobsi]

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Fixes #<issue_number_goes_here> 🦕

[tswast]

doctest failure seems unrelated to this change:

___________ [doctest] indexes.accessor.DatetimeProperties.dayofyear ____________
[gw4] linux -- Python 3.12.7 /tmpfs/src/github/python-bigquery-dataframes/.nox/doctest/bin/python
072 
073         **Examples:**
074 
075             >>> import pandas as pd
076             >>> import bigframes.pandas as bpd
077             >>> bpd.options.display.progress_bar = None
078             >>> s = bpd.Series(
079             ...     pd.date_range('2016-12-28', '2017-01-03', freq='D').to_series()
080             ... )
081             >>> s.dt.dayofyear
Differences (unified diff with -expected +actual):
    @@ -7,3 +7,2 @@
     2017-01-03 00:00:00      3
     dtype: Int64
    -dtype: Int64