nixos/slskd: fix documentation link formatting

        Web interface credentials should also be set here in `SLSKD_USERNAME` and `SLSKD_PASSWORD`.

        Other, optional credentials like SOCKS5 with `SLSKD_SLSK_PROXY_USERNAME` and `SLSKD_SLSK_PROXY_PASSWORD`

        should all reside here instead of in the world-readable nix store.

        Variables are documented at https://github.com/slskd/slskd/blob/master/docs/config.md

        Variables are documented at <https://github.com/slskd/slskd/blob/master/docs/config.md>

      '';

    };