Update the parameter documentation in curve_fit

The documentation describing the `sigma` parameter of the
`optimize.curve_fit` function was updated to remove the reference
to representing the standard deviation of the data.  This is 
misleading because the `sigma` value is used purely as a relative
weighting factor.

See also:
http://mail.scipy.org/pipermail/scipy-user/2013-February/034191.html
Merge pull request #446 from taldcroft/curve-fit-sigma-doc

Update the parameter documentation in curve_fit