Technically, "a small knife", yes, but it's a terrible idea. On page 160 of the Player's Handbook (PHB), the list of trinkets includes a "small knife (#32), which means you could throw it as an improvised weapon. Now, why you would want to do that is another question. Casting Prestidigitation takes up 1 action, so you would need to use another action (normally wait for your next turn) to use the improvised weapon. You would have the same effect by simply picking up a rock and lobbing it at your enemy, which would not cost you the action. It is also worth keeping in mind that the range of the spell is 10 feet, meaning it would dispel out of this range.