Fix typo

Author: Kyotaro HORIGUCHI
Discussion: https://postgr.es/m/20180307.163428.209919771.horiguchi.kyotaro@lab.ntt.co.jp

src/backend/utils/misc/guc.c

@@ -2756,7 +2756,7 @@ static struct config_int ConfigureNamesInt[] =
 
 	{
 		{"max_parallel_workers", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
-			gettext_noop("Sets the maximum number of parallel workers than can be active at one time."),
+			gettext_noop("Sets the maximum number of parallel workers that can be active at one time."),
 			NULL
 		},
 		&max_parallel_workers,