DOC: Fix delim_whitespace regex typo.
Minor typo in the explanation of delim_whitespace which tripped up a user on SO (although the user should probably have been using `delim_whitespace=True` directly anyhow.)

@dsm054 I see that the same typo has been made in the read_csv docstring. Can you fix it there as well?
 @jorisvandenbossche: added the read_csv fix and we're green.
 Thanks!
