If its under the Ortlieb five year warrantee or you want it professionally done, send it back to them: https://www.ortlieb.com/en/service/garantie/
Locally, a local mountain shop, seamstress/tailor, sailmaker, upholstery shop, or shoe repair shop (tip of the hat to Daniel Hicks and dlu for the additions) might be able to do it for you. Whoever does the repair will need an industrial sewing machine -- or, more likely will have to handsew it because of its short length and awkward location. Prices will vary accordingly.
Otherwise, you could yourself sew it back together with Kevlar thread and a large sewing needle or awl, with perhaps some ballistic nylon as backing. Since the tear is at a corner, you may have to dismantle the pack a bit. If this paragraph makes no sense, don't attempt it.
A quicker and much, much dirtier fix would be to use repair tape: either tent repair tape, resin-impregnated fiberglass or cloth tape (such as fiberfix), or make your own using epoxy resin and ballistic cloth. Or, as always, duct tape.