[PATCH] pgbench: fix incorrect long option name in error message

Hi hackers,

While experimenting with pgbench I passed a non-numeric value to -c
and got:

pgbench -c abc
pgbench: error: invalid value "abc" for option -c/--clients

The message refers to --clients, but no such option exists -- the
actual long option is --client (singular), as shown in --help and in
the documentation. Following the message's suggestion fails:

pgbench --clients=10
pgbench: illegal option -- clients

All the sibling option_parse_int() calls in pgbench.c spell the
option name correctly (-j/--jobs, -T/--time, ...); this one looks
like a simple typo. I verified it is present on master and in
released versions (18.4).

The attached patch fixes the message and updates the TAP test in
002_pgbench_no_server.pl that was asserting the wrong spelling.

Regards,
Semih Doğan