Update workflow_by_code.ipynb (#697)

changed model_strategy to signal_stragey. there is no model_strategy under qlib.contrib.strategy.

Author: wan9c9

examples/workflow_by_code.ipynb

@@ -204,7 +204,7 @@
     "    },\n",
     "    \"strategy\": {\n",
     "        \"class\": \"TopkDropoutStrategy\",\n",
-    "        \"module_path\": \"qlib.contrib.strategy.model_strategy\",\n",
+    "        \"module_path\": \"qlib.contrib.strategy.signal_strategy\",\n",
     "        \"kwargs\": {\n",
     "            \"model\": model,\n",
     "            \"dataset\": dataset,\n",