If it was nutrient burn it would be all over. My guess would be that maybe some water splashed on the leaf right there and burned it a bit. When you water always be sure not to leave any of the leafs wet under the light, the water acts like a magnifying glass. I always raise my lights way way way up, water, then direct airflow over them to dry the leafs, and once they're dry drop the light back on them.
My advice is to keep an eye on it, if the problem starts showing up on other leafs or progressing at all you've got a problem. Throughout the grow leafs get damaged. It's good to be proactive about real issues, but when you get the hang of it, more often than not, leaf damage is environmental.