Revert "Fix aggregation notebook"

Author: melonora

notebooks/examples/aggregation.ipynb

@@ -143,7 +143,7 @@
     "    # 10X reaction / sample / batch\n",
     "    # batch_key='dataset',\n",
     "    # cell type, covariate used for constructing signatures\n",
-    "    labels_key=\"celltype_major\",\n",
+    "    labels_key=\"celltype_major\"\n",
     "    # multiplicative technical effects (platform, 3' vs 5', donor effect)\n",
     "    # categorical_covariate_keys=['Method']\n",
     ")"