comment fix in postmaster.c

While I worked on a patch, I noticed a comment that is inconsistent
with the fact.

> * SIGQUIT is the special signal that says exit without proc_exit
> * and let the user know what's going on. But if SendStop is set
> * (-s on command line), then we send SIGSTOP instead, so that we
> * can get core dumps from all backends by hand.

SendStop is set by "-T" option. It was changed by 86c23a6eb2 from "-s"
in 2006.

The attaches fixes the comment for the master branch.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center