You may have encountered a mild chemical burn due to the acidic nature of the lemon juice. This may have led to mild irritant contact dermatitis. You should try to keep the area clean and use anti-bacterial soap. Application of some moisturizers or emollient creams will be helpful in softening the skin and reducing the scar over a period of time.
You may also consult a dermatologist for some antibiotic cream (to prevent secondary infections by staph) or topical corticosteroid, which will help in faster healing of the pigmented scar.