It depends on what you want to stress. If you only mean a left-right flipped S shape, I think it is usually called a "negative sigmoid" (example 1 on p.4, example 2, example 3 on p.4). However, logistic-like functions with negative parameters such as $1/(1+e^{-(-x)})$ are also called negative sigmoids. Certainly, such functions can be written in the form of $\mathtt{constant} + \mathtt{negative\ multiple} * \mathtt{some\ sigmoid}(x),$ but the stress here is the shape of the curve rather the negative multiple itself.
If you want to stress that the function is constructed by multiplying a sigmoid function by a negative number, I don't know the proper name, but I agree with the others here that "negated sigmoid" is a good name. Yet, a google search on "negated sigmoid" returns only 25 results. So this is perhaps not a very common name.