pgbench: Fix order in --help output

The new option --progress-timestamp was just added at the end.  Put it
in alphabetical order.

src/bin/pgbench/pgbench.c

@@ -438,8 +438,8 @@ usage(void)
 		 "  -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"
-		   "  --sampling-rate=NUM      fraction of transactions to log (e.g. 0.01 for 1%%)\n"
 		"  --progress-timestamp     use Unix epoch timestamps for progress\n"
+		   "  --sampling-rate=NUM      fraction of transactions to log (e.g., 0.01 for 1%%)\n"
 		   "\nCommon options:\n"
 		   "  -d, --debug              print debugging output\n"
 	  "  -h, --host=HOSTNAME      database server host or socket directory\n"