Update for failing doc_test.

Author: hansthen

Doc/library/argparse.rst

@@ -1146,6 +1146,9 @@ By default, when a user passes both a ``type`` and a ``choices`` argument, the
 This can cause confusing ``usage`` and ``help`` strings.
 To specify ``choices`` before conversion, set the flag ``convert_choices``.
 
+   >>> def to_dow(s):
+   ...     return ['mo', 'tu', 'we', 'th', 'fr', 'sa', 'su'].index(x)
+   ...
    >>> parser = argparse.ArgumentParser()
    >>> parser.add_argument('when',
    ...                     choices=['mo', 'tu', 'we', 'th', 'fr', 'sa', 'su'],