correct help message

BUG=aomedia:451

Change-Id: I5636c1993a496ddff5d27afa7e72dd787276a86c
diff --git a/aomenc.c b/aomenc.c
index 2d0836e..224c9c8 100644
--- a/aomenc.c
+++ b/aomenc.c
@@ -365,7 +365,7 @@
 
 #if CONFIG_AV1_ENCODER
 static const arg_def_t cpu_used_av1 =
-    ARG_DEF(NULL, "cpu-used", 1, "CPU Used (-8..8)");
+    ARG_DEF(NULL, "cpu-used", 1, "CPU Used (0..8)");
 static const arg_def_t tile_cols =
     ARG_DEF(NULL, "tile-columns", 1, "Number of tile columns to use, log2");
 static const arg_def_t tile_rows =