DELTA_Q: use correct probability description.

Delta Q is coded with a single tree context.

Change-Id: Ie1e56c70285e948232049bfb4a51ad0e9ee25f75
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 3198f89..e4f464b 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -588,7 +588,7 @@
 #else
   const int probwt = 1;
 #endif
-  for (k = 0; k < DELTA_Q_CONTEXTS; ++k) {
+  for (k = 0; k < DELTA_Q_PROBS; ++k) {
     av1_cond_prob_diff_update(w, &cm->fc->delta_q_prob[k], counts->delta_q[k],
                               probwt);
   }