Fix some section levels

Author: ageron

12_custom_models_and_training_with_tensorflow.ipynb

@@ -5866,7 +5866,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# 12. Implement a custom layer that performs _Layer Normalization_\n",
+    "## 12. Implement a custom layer that performs _Layer Normalization_\n",
     "_We will use this type of layer in Chapter 15 when using Recurrent Neural Networks._"
    ]
   },

13_loading_and_preprocessing_data.ipynb

@@ -2693,7 +2693,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Using Feature Columns for Parsing"
+    "## Using Feature Columns for Parsing"
    ]
   },
   {

14_deep_computer_vision_with_cnns.ipynb

@@ -1900,7 +1900,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Mean Average Precision (mAP)"
+    "## Mean Average Precision (mAP)"
    ]
   },
   {

15_processing_sequences_using_rnns_and_cnns.ipynb

@@ -122,7 +122,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Generate the Dataset"
+    "## Generate the Dataset"
    ]
   },
   {
@@ -228,7 +228,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Computing Some Baselines"
+    "## Computing Some Baselines"
    ]
   },
   {
@@ -444,7 +444,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Using a Simple RNN"
+    "## Using a Simple RNN"
    ]
   },
   {