update : index.html : Default user name

The default username is `root`, as of [GitLab's README](https://github.com/gitlabhq/gitlabhq#run-in-production-mode).

Just tested and stumbled across this while fireing up the oneliner from [the page](http://www.damagehead.com/docker-gitlab/).
Merge pull request #147 from almereyda/patch-1

update : index.html : Default user name