Remove the word "would". This isn't replacing milling yourself, it's triggering off milling yourself.

Probably better also to say "Whenever you put one or more cards from your library into your graveyard, target opponent..."

If you did want this to prevent you being milled, it'd have to instead say

> If one or more cards would be put into your graveyard from your library, instead each opponent puts that many cards from the top of his or her library into his or her graveyard.

Replacement abilities can't target, so it'd need to be "each" opponent in that case.