The Infernal Legacy trait grants a Tiefling the ability to cast Hellish Rebuke as a 2nd level spell and the Players Handbook says that you use Charisma as the spellcasting ability modifier of these spells.

So if my Ranger Tiefling has a Charisma score of 14 would the Spell Save DC for Hellish Rebuke's damage be 10 (8+Charisma Mod of 2) or would I add the proficiency bonus to it as well?