services.postgres: add initialScript example

      initialScript = mkOption {

        type = types.nullOr types.path;

        default = null;

        example = literalExpression ''

          pkgs.writeText "init-sql-script" '''

            alter user postgres with password 'myPassword';

          ''';'';

        description = lib.mdDoc ''

          A file containing SQL statements to execute on first startup.

        '';