I think it is always best to go over in person. Just NICELY explain that you'd really appreciate it if they could park the car just a bit up from where it is.
If these are your neighbors, you will be well-served to build a civil, if not outright friendly relationship with them. Go over and make nice, explain the situation politely, you'll likely get a more positive response than just sticking a note on their door. That could be interpreted as disrespectful or cowardly.