DOC: Fix typo in _iotools.py docstring.
DOC: Fix typo in _iotools.py docstring.

The error is in the StringConverter.upgrade docstring.

andsor/numpy,NextThought/pypy-numpy,andsor/numpy,NextThought/pypy-numpy,andsor/numpy,andsor/numpy,NextThought/pypy-numpy,NextThought/pypy-numpy