Change link to WrappedUnions.jl in README

[Tortar]

Hi @MasonProtter, I remember that you pushed for a name like this in the past :-)

LightSumTypes.jl is still named the same, but I created a new package (https://github.com/Tortar/WrappedUnions.jl) which would have been a big breaking change for that library to be done appropriately, and I would like to change the link to that one instead of the previous one, because I think it has a much better interface for Julia in my opinion.