Fix for suspected typo in flexibility.md

[Bobulous]

Corrected what I believe to be copy-pasta in the C-CALLER-CONTROL guideline.

[Bobulous]

I think the original wording is intentional. This guideline is discussing functions that do not do anything that needs ownership of the argument, only dealing with the argument by borrow.

Could you find a different way to reword this?

Oh, so the original comment means "use b as if it's been borrowed, but take ownership and drop it before function returns"?

I understand now, but I did read it (and re-read it) and think it was a typo. Might just be my brain tripping over it, though. If no one else was baffled by it then it might be fine the way it is.