Fix import from SVN link
Zendesk issue: https://gitlab.zendesk.com/agent/tickets/10124
Customer reports: "When you choose to import project from "git Any repo by URL" the help text you are given ends with "To migrate an SVN repository, checkout this document.", where "this document" is a link to http://doc.gitlab.com/ce/workflow/migrating_from_svn.html, which gives a 404."
This merge request corrects the link.