Adding a short hint to *:80 for default listen address on nginx example config

Based on posting https://github.com/gitlabhq/gitlabhq/issues/3384#issuecomment-15457287 I was able to solved an issue here. Pointing out *:80 might could prevent others for struggeling here
Merge pull request #3903 from frlan/master

A small update of nginx example config to prevent users from issues while nginx not listen on localhost