Fix docs

Author: hansthen

Doc/library/argparse.rst

@@ -621,7 +621,7 @@ By default, when a user passes both a ``type`` and a ``choices`` argument, the
 ``choices`` need to be specified in the target type, after conversion.
 This can cause confusing ``usage`` and ``help`` strings. If the user would like
 to specify ``choices`` in the same vocabulary as the end-user would enter them,
-this feature can be enabled by setting ``convert_choices`` to ``True``.
+this feature can be enabled by setting ``convert_choices`` to ``True``::
 
    >>> parser = argparse.ArgumentParser(convert_choices=True)
    >>> parser.add_argument('when',