Tell supervisor I am unhappy with grading even though grades won't change
Unless this decision is somehow going to jeopardize your whole future and not just this one grade then I would suggest to "swallow your pride." You mentioned that you are interested in working with this supervisor in the future. That would certainly be great networking which could bring you a number of opportunities and I wouldn't say risking all of that is worth contesting this one grade (which you may still not be in the right). Leave your pride at the door and reach for your goals only until sometime truly disrespects you. Otherwise try to focus on the positives.