Fix typo in ALTER SYSTEM output.

The header comment written into postgresql.auto.conf by ALTER SYSTEM
should match what initdb put there originally.

Feike Steenbergen

Discussion: https://postgr.es/m/CAK_s-G0KcKdO=0hqZkwb3s+tqZuuHwWqmF5BDsmoO9FtX75r0g@mail.gmail.com