If it was a water based marker I think you have nothing to worry about, the permanent markers have more toxic chemicals but my thoughts are your rabbit will probably be ok because there would not have been enough on the paper to cause problems. I think all you can do is encourage extra water drinking/fluids by giving a bowl of water and feed plenty of hay and leafy greens to flush it out of the system. And then carefully monitor your rabbit and check his poos and urine. If he shows any symptoms of illness take him to the vet with the marker.