A tail that isn't clefted down to the base will not cause any discomfort or harm to the fish. It simply means this is how your fish was born and will only cause the tail be less expanded.
I have a goldie named Copper Kate who, when I bought her, seemed to have a clefted tail down to the base. But it turns out that one of the other fish in the tank had only ripped it down the middle! Within a few months it had sealed right back up again. This would undoubtably happen if you attempted to cleft the tail yourself. Goldfish are animals, not craft projects; yours would be uncomfortable to say the least, as was already addressed by previous posters. The tail fin would have a high risk of getting an infection as well.
Most pet store goldfish are culls, meaning that the breeders only keep fish that have ideal breeding standards, then sell the rest to chain stoes or small shops. These fish make wonderful pets and can have beautiful coloring and finnage, they just wouldn't have the proper requirements to be a show goldfish. It is only important to have a goldfish with perfect confirmation if you are intending to breed that fish to produce show-quality specimens. So just love your fishy the way it is.
You really do learn not to care what they look like after a while, you just love them for their little personalities!
Nossie: I have seen a video of a person trimming a goldfish's headgrowth! The person first sedates the goldfish by letting it swim around in water treated with clove oil. The goldfish eventually passes out. The person then lays them out on a clean surface and proceeds to trim the wen using surgical scisors. Every so often he puts the goldfish back in the water so it doesn't dry out. After the trimming is complete, he puts the fish back into regular water and moves the fish around to get him back to normal. So cool!