Lowering the voltage range of a variable DC bench power supply
Voltage is voltage. If your supply is set for 28 volts, it will provide 28 volts. The current setting is a limit, not a supply setting. That is, a 6 amp supply can provide 5 amps if needed, but it won't force 6 amps through a 5 amp circuit. So don't worry about the mismatch. If things were the other way around, for instance if your supply could only provide 5 amps but the load needed 6, you'd be in trouble. 

So, the supply as specified will do just fine. Don't worry about it.

As a note after watching the video, a couple of points. 1) Don't use a pair of needle-nose to tighten nuts. Get a nut driver or adjustable wrench. 2) Do not get into sloppy habits about soldering like he does with his switch wiring. Always wrap the wire around the terminal  you're soldering to. If you have to hold the wire while you're soldering, you're doing it wrong. The reason this is important is his power demonstration. Remember how the resistors just dropped off the wire when they got too hot? If something goes wrong and your switch wires get hot, you don't want them flopping around the inside of the box.