Bad move. You have not removed a drain/fill plug, you have removed the access plug for one of the gear detents instead. As these sometimes have thrust washers inside the transmission, which now may have dropped, you have a problem as if that washer gets up into the gears, your gear box will promptly become a boat anchor.
These are the correct plugs for servicing the gear oil:
This photo shows a typical six-speed Carrera transmission with both the drain plug (insert upper right, yellow arrow) and fill plug visible (blue arrow). You only need a 10mm hex socket in order to remove either plug (insert upper left).