Apostrophe difference (Ones' and Two's Complement)
Grammatically all the following are correct:
Two's complement = complement of a single 2
Twos' complement = complement of a number of twos (plural)
One's complement = complement of a single 1
Ones' complement = complement of a number of ones (plural)
The wikipedia articles describe two's complement and ones' complement as 2 different mathematical operations. Two's complement is related to a single 2 (to the power of another number) while ones' complement is related to a number of 1s so both are correct.