compress-drv: correct comment

  ''

    mkdir $out

    # cannot use lndir here, because it also symlinks directories,

    # which we do not need; we only need to symlink files.

    # cannot use lndir here, because it stop recursing at symlinks that point to directories

    (cd ${drv}; find -L -type d -exec mkdir -p $out/{} ';')

    (cd ${drv}; find -L -type f -exec ln -s ${drv}/{} $out/{} ';')