Why change the doc root for apache? It sounds like you really need to read the nagios documentation again. Nowhere in the nagios docs, do they tell you to CHANGE the apache doc root. The docs ask you to make an addition, not a change.
Seriously, we could just give you the answer and life would be good, but what good does that do you? Nuttin, since you really need to understand that the nagios documentation is not just an extra, but it's mandatory reading and understanding.
Please make the changes that the docs suggest, and all will be good.