Phase shift is simply the new start point for a graph. For example the sine graph starts at its center and then the wave goes up. So if we want to think of the following two graphs as shifted sine graphs, we would say the first one is shifted -π/4, since that is the x-value that looks like what is normally at the origin for the sine graph. Likewise we would say the second graph is shifted -1/2.
Thus if your answer for the phase shift of the second graph were in terms of π, it wouldn't make any sense since there is no π on the scales of the x-axis in this problem.