Dastiyasha@yandex.com, Have it corrected. I-20 is an important document and any error will cause you problems.
Dastiyasha@yandex.com, Please confirm with the univ if it’s an error. Usually univ provide buffer period to complete the course and provide the end date based on that. Few universities also include opt period in the course duration. So I’m assuming it’s not an error in i20.