Fix obsolete comment - Mailing list pgsql-hackers

Hi.

A comment in ExecUpdate refers to an argument of ExecConstraints that no
longer exists.  Attached fixes that, but I'm about over a year too late
[1] in sending this patch.

Thanks,
Amit

[1]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=c0a8ae7be392