fix workflow_config_lightgbm_multi_freq.yaml (microsoft#635)

Author: zhupr

examples/benchmarks/LightGBM/workflow_config_lightgbm_multi_freq.yaml

@@ -36,13 +36,16 @@ port_analysis_config: &port_analysis_config
             topk: 50
             n_drop: 5
     backtest:
-        limit_threshold: 0.095
+        start_time: 2017-01-01
+        end_time: 2020-08-01
         account: 100000000
         benchmark: *benchmark
-        deal_price: close
-        open_cost: 0.0005
-        close_cost: 0.0015
-        min_cost: 5
+        exchange_kwargs:
+            limit_threshold: 0.095
+            deal_price: close
+            open_cost: 0.0005
+            close_cost: 0.0015
+            min_cost: 5
 task:
     model:
         class: LGBModel