Fix doc for install_tree (cut/paste error)
Fix doc for install_tree (cut/paste error)

It looks like the docs for copy_tree were cut/paste from copy and still referred to installing a "file".

This fixes that.
skosukhin/spack,skosukhin/spack,TheTimmy/spack,tmerrick1/spack,matthiasdiener/spack,EmreAtes/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,mfherbst/spack,matthiasdiener/spack,EmreAtes/spack,iulian787/spack,LLNL/spack,lgarren/spack,tmerrick1/spack,matthiasdiener/spack,tmerrick1/spack,TheTimmy/spack,lgarren/spack,LLNL/spack,matthiasdiener/spack,tmerrick1/spack,krafczyk/spack,mfherbst/spack,EmreAtes/spack,mfherbst/spack,skosukhin/spack,mfherbst/spack,matthiasdiener/spack,skosukhin/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,EmreAtes/spack,krafczyk/spack,TheTimmy/spack,TheTimmy/spack,EmreAtes/spack,krafczyk/spack,lgarren/spack,lgarren/spack,skosukhin/spack,iulian787/spack,tmerrick1/spack,iulian787/spack,TheTimmy/spack,krafczyk/spack,lgarren/spack