nixos/filesystems: remove rotten default value comment for `fsType` (#513481)

        entry in the list is an attribute set with the following fields:

        `mountPoint`, `device`,

        `fsType` (a file system type recognised by

        {command}`mount`; defaults to

        `"auto"`), and `options`

        {command}`mount`), and `options`

        (the mount options passed to {command}`mount` using the

        {option}`-o` flag; defaults to `[ "defaults" ]`).