Fixed a comment on the compound ref frames.

Change-Id: I77e397ac9f594c9c4c1db442e334a6ea5f53f588

vp9/encoder/vp9_rdopt.c

@@ -2488,9 +2488,8 @@ static int64_t handle_inter_mode(VP9_COMP *cpi, MACROBLOCK *x,
   }
 
   // We don't include the cost of the second reference here, because there
-  // are only three options: Last/Golden, ARF/Last or Golden/ARF, or in other
+  // are only two options: Last/ARF or Golden/ARF; The second one is always
-  // words if you present them in that order, the second one is always known
+  // known, which is ARF.
-  // if the first is known.
   //
   // Under some circumstances we discount the cost of new mv mode to encourage
   // initiation of a motion field.