Issue #23651: Fix typo in allow_abbrev docs.

Noticed by Nathan West.

Doc/library/argparse.rst

@@ -538,7 +538,7 @@ This feature can be disabled by setting ``allow_abbrev`` to ``False``::
    >>> parser = argparse.ArgumentParser(prog='PROG', allow_abbrev=False)
    >>> parser.add_argument('--foobar', action='store_true')
    >>> parser.add_argument('--foonley', action='store_false')
-   >>> parser.parse_args([--foon])
+   >>> parser.parse_args(['--foon'])
    usage: PROG [-h] [--foobar] [--foonley]
    PROG: error: unrecognized arguments: --foon