I'm not too familiar with the quote and its origin, so I can't really say it amused me. But a little research showed me that it has something to do with the "I Can't Believe It's Not Butter" product. And as my research confirmed, there's indeed a grammatical error in your card. Try to review it once more, and you'll see that error.

I suggest that you edit the card to correct the statement, because small grammatical errors such as this might decrease the expected level of amusement for this card.