im sure someone in the world knows how. but i can offer an alternative. instead of taking fabric off of the shirt, you could try adding fabric to make the neckhole smaller. i dont know how it will look on the shirt you want to shrink. but im sure you have seen those t shirts that look like they have another shirt underneath but really it is just extra fabric extending the sleeves. the same method can be applied to the neckhole or any fabric hole in general.
Shrinking the neck slightly can also be done by ironing the neck using steam IF the fabric allows for ironing. Simply check the label to insure it won’t damage the fabric.