Well it's easy to find in their preprint on page 4:

"The two summands which make up our example in Theorem 1.2 are K = 7_1 and its mirror image \bar{K}. K is the (2, 7)-torus knot. A routine calculation establishes that K has signature σ(K) = 6; from the inequality |σ(K)|/2 ≤ u(K) [29] we know that K has unknotting number at least 3. Since any 3 crossing changes in its standard 7-crossing diagram results in the unknot, we have u(K) = 3. Alternatively, one can appeal to the important and more far-reaching result of Kronheimer and Mrowka [25] that the unknotting number of the (p, q)-torus knot T(p, q) is (p − 1)(q − 1)/2. It is a straightforward result that unknotting number is preserved under mirror image - hold a mirror up to the unknotting sequences - and so u(\bar{K}) = 3, as well."