Option 1 – Leave it alone – it’ll remind you to drill from both sides in future!
(Reality: this often happens. don’t worry about it….. Acers like maple and Sycamore can be brittle around cut holes and can often shed small bits – it is in the tree’s nature to do this in revenge for cutting it down).
Option 2 – Finish your leg. Wedge and glue it, pare off all the bits then get a well-set card scraper with an aggressive cut and scrape all round it from all directions, finishing with a lighter scraper and probably sandpaper. Most of it, if not all of it, will disappear and you won’t notice the slight depression.
It ain’t what goes wrong…. the important part is how you put it right – that’s what makes us skilled.