You can. Not while turn-by-turn is active (presumably because Apple doesn't want to distract drivers), but outside of that, hit the (i) button and "Report an Is

You can. Not while turn-by-turn is active (presumably because Apple doesn't want to distract drivers), but outside of that, hit the (i) button and "Report an Issue" is an option. What's neat about this is if you report a map error and they correct it, you actually get a push notification telling you that it's been fixed.