I haven't heard that version before, but it may be true, of course. The version I heard is that the English cut off the two fingers of French archers taken prisoners, and that later, when these prisoners were exchanged for English soldiers, the English gave the two-fingered salute as soon as the prisoner swapping was finished, shouting "shoot at as now, wankers/frogs" or something like that. Anyway, it's a cool story (except for the French archers, of course).