fixed threeAreNotInTopRow function

Hi,

I made a change to the `threeAreNotInTopRow` function. I think the correct interpretation of this rule is that for the digits `[a,b,c,d]`, there are three of them (perhaps a,b and c, perhaps some other three) which are not in the top row. This is the same as saying that at most one of the digits are in the top row.

With this change you get a unique solution: "8634" - otherwise you get three solutions.

Regards,
ER