What did you do!?!?!? J/k, relax, all will be fine. If such slight damage killed that seedling then I wouldn't be to bent out of shape about it. You don't want a plant/genetics that couldn't take that damage, even at such a young age.
It's all good though, that damage is minimal. The plant wants to survive. Imagine if they couldn't take damage like that in the wild? The species would of been extinct long ago.