Minor patch; missing comment update in worker.c

Dear hackers,
(Adding Amit in CC because he was an original committer)

While reading codes in master, I found the below comment in worker.c.:
```
* We don't allow to toggle two_phase option of a subscription because it can
* lead to an inconsistent replica. Consider, initially, it was on and we have
* received some prepare then we turn it off, now at commit time the server
* will send the entire transaction data along with the commit. With some more
* analysis, we can allow changing this option from off to on but not sure if
* that alone would be useful.
```

But this is not correct anymore, 1462aad2 allows to alter two_phase option.
I was an original author, but I did oversight.

I feel it can be fixed by referring the commit message, attached patch fixed like
that. How do you feel?

Best regards,
Hayato Kuroda
FUJITSU LIMITED