fix small sample correction of var(Chao) when there are no doubletons

I reconstructed the variance formula (3b) of Chiu et al. 2014 using
the procedure described in their online appendix. I could get the same
formula except for the small-sample correction: it seems that EstimateS
web page got this correctly, but Chiu et al. ignore the small-sample
correction for all but the first term.
Update diversity vignette for the current state of Chao estimates