Those kings of things don't bother me much and I would probably just leave it be. But if you want to fill the gap without refitting the butt plate (which can be tricky) get a pack of those wax blend sticks made by Dap and see how well you can match the color of your stock. If you get a decent match it will be hardly noticable.

http://www.dap.com/product_details.aspx?product_id=360