pgoutput: comment atop rel_sync_cache_publication_cb() has an inconsistency

Dear hackers,

While reading the code, I found $SUBJECT. It says:

```
/*
 * Publication relation/schema map syscache invalidation callback
 *
 * Called for invalidations on pg_publication and pg_namespace.
 */
static void
rel_sync_cache_publication_cb(Datum arg, int cacheid, uint32 hashvalue)
```

But this won't be called for invalidating pg_publication. It should have been
updated by 3abe9dc1, but it was an oversight.

Best regards,
Hayato Kuroda
FUJITSU LIMITED