Move installmirrors anchor in doc

so it's before "Mirrors" instead of "Docker and local DNS server
warnings"

Docker-DCO-1.1-Signed-off-by: Marc Abramowitz <msabramo@gmail.com> (github: msabramo)
Merge pull request #4924 from msabramo/move_installmirrors_anchor_in_docs

Move installmirrors anchor in doc