Fix docstring for na_values in parsers
I discovered entirely by accident that the `na_values` in `pd.read_*` can be a string. According to the docstring, it can only be a sequence or a dict. This PR fixes that.

thanks!
 @jaidevd There are some other places in the docs were this is described (eg in io.rst). Would you like to adapt this as well?
 @jorisvandenbossche Sure. Give me a couple of hours.
 @jorisvandenbossche https://github.com/pydata/pandas/pull/10488
