av1/arg_defs: use a valid target level index in help msg

previously the index used was 21 = 7.1, which is currently undefined

Bug: aomedia:3184
Change-Id: Idc7fe561ecf25b08c2f88086f7aa2c87ff8c0b58
(cherry picked from commit 905abbb42334d7688cdb91315136d456a5e6b5fc)

av1/arg_defs.c

diff --git a/av1/arg_defs.c b/av1/arg_defs.c
index b5f4303..a6b5aab 100644
--- a/av1/arg_defs.c
+++ b/av1/arg_defs.c
@@ -574,7 +574,7 @@
       "AB: Operating point(OP) index, "
       "xy: Target level index for the OP. "
       "E.g. \"0\" means target level index 0 for the 0th OP, "
-      "\"1021\" means target level index 21 for the 10th OP."),
+      "\"1019\" means target level index 19 (6.3) for the 10th OP."),
   .set_min_cr = ARG_DEF(
       NULL, "min-cr", 1,
       "Set minimum compression ratio. Take integer values. Default is 0. "