karakeep: fix broken docs link (#489122)

          Environment variables to pass to Karakaeep. This is how most settings

          can be configured. Changing DATA_DIR is possible but not supported.

          See <https://docs.karakeep.app/configuration/>

          See <https://docs.karakeep.app/configuration/environment-variables>

        '';

        type = lib.types.attrsOf lib.types.str;

        default = { };