It sounds like there is a clerical error in your Final Decree. That is the problem with the routine use of forms. Often all of the provisions of one person's paperwork are not changed to make it conform to the current case. Obviously, there was an agreement to pay support until age 21 if the child was in college and that was made a part of the Final Decree. Trying to rely on a clerical error will not be successful in avoiding that obligation.
It looks like from your question that you signed a settlement agreement that provided for child support to be paid until age 21/college. A provision like this exceeds what the statute provides, but people are able to contract greater obligations if they choose.
Then, your settlement agreement was made part of your divorce decree. Most likely, whoever drafted the divorce decree used the "standard" language of child support ending at age 18, which contradicts the terms of your agreement.
Most likely, the greater obligation of the agreement will prevail against the language of your decree.