Since you went with 10/2 for a 240v outlet that means your cable will have two hots and a single ground. But the cable you bought will have a black, white and bare/green wire. Make sure that you wrap both ends of the white conductor with red electrical tape to indicate that it is also a hot wire. White normally indicates a neutral, but since you won’t have a neutral wire in this cable, it’s important to mark it. Otherwise someone else may work on this outlet in the future, see a white wire, assume it’s a neutral, wire something up with it as a neutral, and give themselves a pretty bad shock.