I wouldn't bother. When you racked, you will have stirred things up a bit, and this will release some CO2, displacing the air. If you add water, you will dilute the beer, and if you add the water from the tap, you will be adding more oxygen (which is what you are trying to avoid). Next time, make a slightly bigger batch to avoid the problem.
Edit. Great minds think alike (but some think a bit quicker than others).