Fix where beta is calculated for qmode deltaq

Bug=b/256852795

Change-Id: I50171a4a4289c52ca67481d59f024e2914439f6a
diff --git a/av1/qmode_rc/ratectrl_qmode.cc b/av1/qmode_rc/ratectrl_qmode.cc
index 9dfdebc..b11a609 100644
--- a/av1/qmode_rc/ratectrl_qmode.cc
+++ b/av1/qmode_rc/ratectrl_qmode.cc
@@ -1313,7 +1313,7 @@
       double beta = 1.0;
       if (mc_dep_cost > 0 && intra_cost > 0) {
         const double r0 = 1 / frame_importance;
-        const double rk = intra_cost / mc_dep_cost;
+        const double rk = intra_cost / (mc_dep_cost + intra_cost);
         beta = r0 / rk;
         assert(beta > 0.0);
       }