doclifter: 2.17 -> 2.18

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/doclifter/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/qzimyzp6c58i4jmqkmijzicyxzbn3j0x-doclifter-2.18/bin/manlifter had a zero exit code or showed the expected version
- /nix/store/qzimyzp6c58i4jmqkmijzicyxzbn3j0x-doclifter-2.18/bin/doclifter passed the binary check.
- 1 of 2 passed binary check by having a zero exit code.
- 1 of 2 passed binary check by having the new version present in output.
- found 2.18 with grep in /nix/store/qzimyzp6c58i4jmqkmijzicyxzbn3j0x-doclifter-2.18
- directory tree listing: https://gist.github.com/5f26018de7c8c2bbf86953b19997143a
- du listing: https://gist.github.com/b556716e68094c4f1457bbbbe6467d55