Any man with an education knew that the earth is round. They figured it out in ancient Greece and by 1492 it was common knowledge. The misconception that Columbus's journey had any connection whatsoever with a belief in a flat earth is a modern fabrication. This error can still be found in some textbooks so it is not surprising that you would have read it somewhere.
Not many had the education 5 centuries ago. The knowledge from the ancient Greeks was lost to western civilization during the “Dark Ages”. It took a long time afterward to spread the knowledge to the masses.