Use GitLab.com instead of an example

Report https://news.ycombinator.com/item?id=11143463

The example in http://doc.gitlab.com/ce/api/ is gitlab.example.com

Replacing this with gitlab.com would make the situation more clear for people using the SaaS and would make it easier to run a CURL command to an active server.

/cc @axil @dzaporozhets @JobV