fix missing chapter 3 header

03_classification.ipynb

@@ -857,6 +857,13 @@
     "cross_val_score(sgd_clf, X_train_scaled, y_train, cv=3, scoring=\"accuracy\")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Error analysis"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 64,
@@ -2574,7 +2581,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.8"
+   "version": "3.8.2"
   },
   "nav_menu": {},
   "toc": {