Fix minor typo in repository_mirroring.md.

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

What does this MR do?

Replaces a single word in the document. The sentence wasn't grammatically correct before, and appears to be a simple mistake.

Moving docs to a new location?

No.

Notes

This is my first MR with GitLab, so please let me know if I should do anything different in the future.