fix target_ensemble nb

Author: ndharasz

target_ensemble.ipynb

@@ -3458,19 +3458,19 @@
         "# Ensemble predictions together with a simple average\n",
         "validation[\"ensemble_cyrus_victor\"] = (\n",
         "    validation\n",
-        "    .groupby(\"era\")[\n",
+        "    .groupby(\"era\")[[\n",
         "      \"prediction_target_cyrus_v4_20\",\n",
         "      \"prediction_target_victor_v4_20\",\n",
-        "    ]\n",
+        "    ]]\n",
         "    .rank(pct=True)\n",
         "    .mean(axis=1)\n",
         ")\n",
         "validation[\"ensemble_cyrus_teager\"] = (\n",
         "    validation\n",
-        "    .groupby(\"era\")[\n",
+        "    .groupby(\"era\")[[\n",
         "      \"prediction_target_cyrus_v4_20\",\n",
         "      \"prediction_target_teager_v4_20\",\n",
-        "    ]\n",
+        "    ]]\n",
         "    .rank(pct=True)\n",
         "    .mean(axis=1)\n",
         ")\n",