Fix a command line help comment

BUG=aomedia:1283

Change-Id: I9b200d8cfb3ffcdd2fb1cece6c54a0f600d37a87
diff --git a/aomenc.c b/aomenc.c
index a040509..f6921eb 100644
--- a/aomenc.c
+++ b/aomenc.c
@@ -457,7 +457,7 @@
 static const arg_def_t qm_min = ARG_DEF(
     NULL, "qm-min", 1, "Min quant matrix flatness (0..15), default is 8");
 static const arg_def_t qm_max = ARG_DEF(
-    NULL, "qm-max", 1, "Max quant matrix flatness (0..15), default is 16");
+    NULL, "qm-max", 1, "Max quant matrix flatness (0..15), default is 15");
 #endif
 #if CONFIG_DIST_8X8
 static const arg_def_t enable_dist_8x8 =