pgbench: Fix help message

Add NUM placeholder to -t option in help message.  It got lost in
79cddb18.

Author: Fabien COELHO <coelho@cri.ensmp.fr>

contrib/pgbench/pgbench.c

@@ -368,7 +368,7 @@ usage(void)
 		   "  -R, --rate=NUM           target rate in transactions per second\n"
 		   "  -s, --scale=NUM          report this scale factor in output\n"
 		   "  -S, --select-only        perform SELECT-only transactions\n"
-		   "  -t, --transactions       number of transactions each client runs (default: 10)\n"
+		   "  -t, --transactions=NUM   number of transactions each client runs (default: 10)\n"
 		   "  -T, --time=NUM           duration of benchmark test in seconds\n"
 		   "  -v, --vacuum-all         vacuum all four standard tables before tests\n"
 		   "  --aggregate-interval=NUM aggregate data over NUM seconds\n"