Why is there a small black line in my MacBook Pro screen?

Those are dead pixels! Often we see a few single ones which with such a dense (pixels per inch) a retina display has are very hard to see.

Given how new your system is I would bring my system in to an Apple Store to get a new display installed under warranty.

But you should do a bit of homework to have the upper hand in the dialog. Very carefully place a fine lined ruler against the line in parallel to it so you can measure the number of pixels effected. You’ll need to take a good tight and sharp picture of the area. Then blow it up so you can count the effected pixels. Don’t volunteer what you found, you want this as your ace in the sleeve. If you get push back, ask them for something written that states what you have is not covered. They should show you the current policy, then you can show your stuff that clearly shows this should be covered. Given the fact the pixels are so grouped its to big a blemish to dismiss.

Reference: Leaked Internal Policy Says How Many Dead Pixels It Takes To Get Apple To Replace A Faulty Display while old it gives you a framework.