Update model_intra_yrd_and_prune() comment

BUG=aomedia:2959

Change-Id: Id0e87b7fb8814e715fd5e9cd30f29a0d9f926287
diff --git a/av1/encoder/intra_mode_search_utils.h b/av1/encoder/intra_mode_search_utils.h
index 7dc5759..94b0dad 100644
--- a/av1/encoder/intra_mode_search_utils.h
+++ b/av1/encoder/intra_mode_search_utils.h
@@ -440,7 +440,7 @@
  * \callergraph
  * This function first makes a quick luma prediction and estimates the rdcost
  * with a model without going through the txfm, then try to prune the current
- * mode if the new estimate y_rd > 1.5 * best_model_rd.
+ * mode if the new estimate y_rd > 1.25 * best_model_rd.
  *
  * \return Returns 1 if the given mode is prune; 0 otherwise.
  */