head・canon / hed kan-uh n /
A term used to describe one's particular idea/opinion about a part of their fandom (character traits, plot details, etc) that is either not verified or completed negated by the original content; but the fan believes and/or likes their idea/opinion so much that, to them, it may as well be canon (officially part of the original content).
"I have this headcanon that the character Bob Belcher, from the television show 'Bob's Burgers', is bisexual."
— A Girl of Many Fandoms
I cannot over emphasize that a headcanon is a personal opinion meant solely to increase one's enjoyment of a fandom, regardless of whether it has any basis in canon or not. Which is why, in this particular instance, personal opinions of this nature can neither be correct nor incorrect; because it's a matter of what makes the person (or persons) who ascribe to the opinion happy. Additionally, in this particular instance, personal opinions of this sort are harmless as they are not meant to be considered canonical by anyone except the person (or persons) who ascribe to the opinion.
Therefore, I would NOT recommend correcting a person's headcanon as it would be considered rude.