I have a switch that controls both halves of an outlet. I want to convert it so that it only controls half the outlet, leaving the other one hot all the time I found a diagram online, does it look right?
It's probably not possible with your configuration. The incoming hot is switched, and there's likely no constant hot source present (unless it was wired as a split initially and someone modified it). You can certainly explore the situation to find out for sure. We can't say from here.
Your diagram shows a 3-wire Romex cable between the switch and the receptacle. Is there a 3-wire cable between the two?
If not, you still would be able to make a half-switched duplex receptacle if there is a line hot in the receptacle box. Is there one? If there is a line hot in the receptacle box, then you have a very easy task which could be done by changing some connections at the receptacle box with little time and effort.
Maybe you envision pulling a new 3-wire cable? If the switch cable does not go through multiple studs to get to the receptacle, then experts might offer hope you could do this. If the receptacle box and/or switch box is fastened to a stud, you'd have to remove the boxes (to reach in and pull out staples), then use the old 2-wire cable to pull a new 3-wire cable. This might be doable if the cable goes up through the top plate into an accessible attic, but might be very hard or impossible if the cable goes through even one hole in a stud.