DOC: update usage of randn in io.rst
In the IO page of the docs (https://github.com/pandas-dev/pandas/blob/master/doc/source/io.rst), there are some occurrences of direct calls to `randn` instead of using the properly imported `np.random.randn` in full, which makes the examples not reproducible if you only imported `import numpy as np`.

A PR that replaces `randn` with `np.random.randn` would thus be very welcome.