Fix #3030

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Fix #3030 (closed)

Notes

This is my first contribution to Gitlab. I've read doc_styleguide and CONTRIBUTING. Please don't hesitate to tell me what I've done wrong :)

I thought I could rename my branch (to follow Writing Documentation#testing naming recommendations) but I didn't find out how in the web UI.

If this first contrib is OK, I think we should also update the documentation embedded in the application (app/views/projects/mirrors/_instructions.html.haml and app/views/shared/_import_form.html.haml)