Mark it correct of course. The correction demonstrates the knowledge we were testing for, and even if they missed the entire point of how True-False is supposed to work, we're ultimately trying to determine their knowledge of course objectives, not their understanding of test-format. So that's just funny, but deserves a mark.
Indeed, I would argue that all True-False should work that way. Good T/F design* includes students explaining why the F options are false. That not only catches the sort of problem above, but ensures that students choosing F aren't just guessing, and that they know the right answer. There may also be more than one way to make a false statement true...that's all okay, as long as it demonstrates the students knowledge (or lack thereof).
T/F Ice freezes at 0 degrees C
That's true; but 'false' and adding "at sea level" is also correct and tells you even more about the student's knowledge. Both answers should get the mark.
*Okay, that's an oxymoron: almost all T/F are useless and to be avoided, but there are a few special circumstances—memorization of definitions, names, or dates, as if any of that is ever useful—where they can be used successfully.
