Television viewers apparently had their fill of football by the time NBC's Sunday Night Football telecast began Sunday night. The Washington-Baltimore match-up attracted 12.56 million viewers, according to overnight Nielsen's -- somewhat below average. The figure was almost half what Fox registered earlier in the evening for its NFL overrun -- 23.45 million. A Hallmark Hall of Fame special, "Front of the Class," competed strongly with the game. Although the drama averaged 11 million viewers during its first hour on Sunday, the ratings climbed with each half hour, peaking with 12.75 million AT 10 30 p.m. for fourth place.