There is an adjustment you can make to the rack preload, but you run the risk that if you tighten it when it's on-center, it'll bind when it's off center. So, turn the wheel half a turn off center and see if it still has the play. If not, don't make any adjustments, you're going to have to live with it until you replace the rack. If there is still slack, look on the front of the rack, opposite the input shaft, and you'll see a big nut and locking ring nut. It might have a cover over it, if so, remove the cover. Loosen the locking ring, and then tighten the big preload nut slightly. Check how much play is left. Frankly, I'd leave a very slight amount to try to ensure it isn't too tight anywhere in its travel. When you think you have it right, turn the wheels lock to lock a few times and see if you feel any tight spots. If so, loosen it back up a little.