Fix doctest issue

Author: hansthen

Doc/library/argparse.rst

@@ -1186,6 +1186,7 @@ To specify ``choices`` before conversion, set the flag ``convert_choices``.
    ...                     choices=['mo', 'tu', 'we', 'th', 'fr', 'sa', 'su'],
    ...                     convert_choices=True,
    ...                     type=to_dow)
+   ...
    >>> parser.print_help()
    usage: example_broken.py [-h] [--days {mo,tu,we,th,fr,sa,su}]