bug in virtual species documentation

The example calls logistic as 

`predictors = [logistic(centers[i], shapes[i]) for i in eachindex(L)]`

but the arguments are flipped as logistic is defined as 

```
logistic(x, α, β) = 1 / (1 + exp((x - β) / α))
logistic(α, β) = (x) -> logistic(x, α, β)
```

so the first argument `α` is the shape and the second argument `β` is the center