update nginx config hint

Created by: darkrasid

When use gitlab with relative url, user should change location regular expression on nginx config file.

Because example of nginx config file contain this type regular expression.

^/[\w\.-]+/[\w\.-]+/(info/refs|git-upload-pack|git-receive-pack)$

it will catch some URL like below.

/project/repo.git/git-upload-pack

However if user set relative url, upper url should be

/relative_url/project/repo.git/git-upload-pack

so regular expression isn't able to catch.

It makes errors when cloing, pulling, pushing or archiving.
(ex. it will clone empty repository, even if the repository does not empty.)

I think it is better way that comment to user in example of nginx config file