One Wise Advice

This is what I've learnt today while reading Aku Ankka he he he...

"Älä jätä huomiseen sitä, minkä voi tehdä tänään."

It means: "Don't put off till tomorrow what you can do today."

At first I was confused why it's written "minkä", but now I get why. It's because if we break them down into two sentences, it'd be like this:

Älä jätä sitä huomiseen. Sen voi tehdä tänään.

Because it's "sen voi tehdä tänään", then the word "mikä" changed into "minkä".

P.S. I'm only learning here, so correct me if I'm wrong!!!