Fix a bug in mean calculation of 'ys_probs' (#748)

@@ -152,7 +152,7 @@ void TransducerKeywordDecoder::Decode(
       if (matched) {
         float ys_prob = 0.0;
         int32_t length = best_hyp.ys_probs.size();
-        for (int32_t i = 1; i <= matched_state->level; ++i) {
+        for (int32_t i = 0; i < matched_state->level; ++i) {
           ys_prob += best_hyp.ys_probs[i];
         }
         ys_prob /= matched_state->level;