Fix the comment in ratectrl.h

Change-Id: Ia99af7153eeb2f4e289b882403d34f7b7c118dc2

av1/encoder/ratectrl.h

diff --git a/av1/encoder/ratectrl.h b/av1/encoder/ratectrl.h
index e3b4184..f8c578c 100644
--- a/av1/encoder/ratectrl.h
+++ b/av1/encoder/ratectrl.h
@@ -207,8 +207,8 @@
   /*!\cond */
 
   // rate control history for last frame(1) and the frame before(2).
-  // -1: undershot
-  //  1: overshoot
+  // -1: overshoot
+  //  1: undershoot
   //  0: not initialized.
   int rc_1_frame;
   int rc_2_frame;