If the reaction occurs in the direction indicated, NADH will reduce acetoacetate -> B-hydroxybutyrate which picks up a proton in the process. This raises the pH.
Based on the reduction potential table, though, the reaction is actually spontaneous in the opposite direction and NADH will be produced instead. The explanation is poorly written as it even states that the reaction will shift to the left, but then says that this results in higher concentrations of B-hydroxybutyrate (a right shift)