MISLEADING ADVERTS ON E-MAIL

I received an e-mail RNR Holidays stating that you can book a weekend or weekday "STAY 3 NIGHTS AND GET 2 NIGHTS FREE" On making the booking, I am now told I have to stay minimum 5 nights to qualify for 2 free nigHts. Nowhere in the advert does it state that. Is this not a form of misleading advertising? Please help