documentation: correct documentation help
documentation: correct documentation help

`PANDOC_LUA_FILTERS_DIR` is set in `makeFlags` in `doc/default.nix`,
and needs to be explicitely passed to `make` when called manually.

Signed-off-by: Pamplemousse <e220f2b14804d9d261bf746623919194cf54cbd2@gmail.com>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs