Fix comment in report_sequence_errors()

Hi

I noticed an incorrect comment in report_sequence_errors() in sequencesync.c.

The function comment describes case (c) as:

```
* c) missing sequences on the subscriber
```

However, the actual warning message emitted and the logic that
populates missing_seqs_idx both refer to sequences that are missing on
the publisher side, not the subscriber.

The fix is to replace "subscriber" with "publisher" in that comment line.

Thanks
Imran Zaheer