The shift cable has an adjustment in it, but it is hard to explain. I recommend watching some YouTube videos of people adjusting it. I read the GM procedure and didn't understand what they were talking about, but after watching some videos, it made sense.
Basically, the cable is in two halves, and there is a threaded sleeve between them. You remove a plastic lock and the two halves will be free to slide around. You just jostle the shifter in the park position a bit, then re-insert the lock, and you're done.
Like I said, watch some videos on YouTube. It is easier to see than to read.