update mirror config documentation.
update mirror config documentation.

- mirrors.yaml uses Spack's OrderedDict rather than lists.

LLNL/spack,TheTimmy/spack,mfherbst/spack,EmreAtes/spack,tmerrick1/spack,EmreAtes/spack,tmerrick1/spack,iulian787/spack,tmerrick1/spack,krafczyk/spack,mfherbst/spack,tmerrick1/spack,iulian787/spack,iulian787/spack,TheTimmy/spack,krafczyk/spack,EmreAtes/spack,iulian787/spack,mfherbst/spack,matthiasdiener/spack,matthiasdiener/spack,mfherbst/spack,lgarren/spack,LLNL/spack,TheTimmy/spack,lgarren/spack,TheTimmy/spack,skosukhin/spack,lgarren/spack,matthiasdiener/spack,matthiasdiener/spack,LLNL/spack,LLNL/spack,skosukhin/spack,lgarren/spack,skosukhin/spack,EmreAtes/spack,krafczyk/spack,skosukhin/spack,lgarren/spack,iulian787/spack,matthiasdiener/spack,TheTimmy/spack,tmerrick1/spack,mfherbst/spack,krafczyk/spack,skosukhin/spack,LLNL/spack,krafczyk/spack,EmreAtes/spack