filesystem.md - Match better to Backdrop

When we first ported to Backdrop, Jen+Nate wanted the default file structure
to be a bit different from Drupal (e.g.  `/civicrm.settings.php` instead of
`/sites/default/civicrm.settings.php`; `/modules/civicrm` instead of
`/sites/all/modules/civicrm`).

I think the Backdrop folks have a slightly different approach toward
multisite than the D7 folks: IIRC, the multisite stuff was still there
functionally, but it wasn't the default.