docs: minor updates

Author: mdtanker

docs/examples/bishop_basement_model.ipynb

@@ -16,7 +16,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": null,
    "metadata": {
     "execution": {
      "iopub.execute_input": "2024-07-22T22:22:21.519589Z",
@@ -45,6 +45,7 @@
     "from polartoolkit import utils as polar_utils\n",
     "\n",
     "from invert4geom import (\n",
+    "    cross_validation,\n",
     "    inversion,\n",
     "    optimization,\n",
     "    plotting,\n",