Actually, you don't have a circumcision mark. That would be a clearly visible scar, stitch marks etc.
What you have is a perfectly natural contrast of 2 different skin types where they join. (while this joining may be due to the circumcision, the colour contrast is just due to different skins being joined togerher. The actual circumcision scar is probably barely visible for you.
Some uncircumcised can have a fairly similar transition from inner to outer skins, while orthers have more gradual transition.