Adding the missing hyphen between git and svn for the Git/SVN mirror link

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

What does this MR do?

This merge request fixes a broken link within the docs/workflow/importing page for git/svn mirror.

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.