Both versions are correct (though I personally prefer the spelling sans 's'.) Guillaume de l'Hôpital's name was originally spelled with an 's', but the French language has evolved and the 's' was dropped in favor of a circumflex accent. In French the use of a circumflex accent is often used to indicate the disappearance of a silent 's'. For example, the words tête (head), fenêtre (window), pâte (pasta), coût (cost), and of course hôpital (hospital) (compare with the Italian testa, finestra, pasta, costo, and ospedale) all use the circumflex in this way.

But his rule is universal, regardless of how we spell it.

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Three people were asked whether it was better to have a wife or a mistress.

Doctor: "A wife, because studies have shown that wives have beneficial effects on your health."
Lawyer: "Definitely a mistress, because a wife will just take all your money in a divorce."
Mathematician: "Both. That way, your wife thinks you're with your mistress, your mistress thinks you're with your wife, and there's more time to do math."