nixos/spacefm: unbreak manual build

Fixup regression introduced in commit 1bbcd91b
("spacefm: sudo and gksu fixes #15758 and license update").

A missing </filename> end tag caused this:

  $ nixos-rebuild build
  ...
  options-db.xml:4402: parser error : Opening and ending tag mismatch: filename line 4401 and para
  </para><para><emphasis>Type:</emphasis> boolean</para><para><emphasis>Default:</
         ^
  options-db.xml:4406: parser error : Opening and ending tag mismatch: filename line 4401 and listitem
              </filename></member></simplelist></listitem></varlistentry><varliste
                                                          ^
  options-db.xml:4406: parser error : Opening and ending tag mismatch: para line 4401 and varlistentry
              </filename></member></simplelist></listitem></varlistentry><varliste
                                                                         ^
  options-db.xml:28430: parser error : Opening and ending tag mismatch: listitem line 4401 and variablelist
         </filename></member></simplelist></listitem></varlistentry></variablelist
                                                                                 ^
  options-db.xml:28432: parser error : Premature end of data in tag varlistentry line 4401