Clarify what number_ios does in the HOWTO/man page/help

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/options.c b/options.c
index 56e9108..3a7c41a 100644
--- a/options.c
+++ b/options.c
@@ -1726,7 +1726,7 @@
 		.lname	= "Number of IOs to perform",
 		.type	= FIO_OPT_STR_VAL,
 		.off1	= td_var_offset(number_ios),
-		.help	= "Force job completion of this number of IOs",
+		.help	= "Force job completion after this number of IOs",
 		.def	= "0",
 		.category = FIO_OPT_C_IO,
 		.group	= FIO_OPT_G_INVALID,