A dealer can usually fix this in about 5 minutes without an appointment if the light doesn't go out after driving a few miles after the tires have been refilled... Mine did it to me at the end of Fall and filling the tires plus 60 miles round trip didn't get the tire warning light to turn off.
2012 Focus Ti Hatch, Blue Candy
2011 Mustang GT, Grabber Blue, 6spd
1978 Mustang II, 351W, Tremec 3550
1978 Mustang II, 2.3L Turbo, 5spd