Well, it depends on what you call zero potential energy. If you say "ball sitting on the floor" is zero potential energy, then $Mgh$ is the correct formula for the potential energy of the ball on the ramp. If you say "ball material flattened onto the floor" is zero, then $Mg(h+R)$ is indeed the correct formula. If you say "ball sitting at the bottom of a 30 meter pit" is zero, then the formula would be $Mg(h+30)$.
None of these zero points is "wrong"; potential energy is always determined relative to some arbitrary zero point. As the problem is written, though, it implies that zero is "ball sitting on the floor", hence $Mgh$ is indeed the expected answer.