Replaced Piece of roof sheathing, forgot to use 1/8th inch space - should I be worried?
The gap is to allow for expansion due to heat and humidity. If you just installed it this summer I wouldn't lose any sleep over it. It is already pretty warm and humid so the wood should be near its maximum dimensions. One sheet out of many is not likely to cause any problems.

If you are really OCD then take a circular saw and set the depth shallow at the thickness of the plywood so you don't cut into the trusses. Then cut along the butt joint with the other sheets. This will create a gap the width of the blade ~1/16".

Good luck!