ToPyObject example incorrect for PyString

[stuhood]

ToPyObject indicates that:

You can always calls val.to_py_object(py).into_py_object() in order to obtain PyObject (the second into_py_object() call via the PythonObject trait corresponds to the upcast from PyList to PyObject).

But this doesn't seem to be the case for PyString, for which the ToPyObject impl targets itself, making the into_py_object call a noop.

[markbt]

You should be able to use into_object() to convert a PyString to a PyObject. (into_py_object() just converts to the downcasted type, which is already PyString).

[markbt]

(I'll update the docs later)