Added few lines of code in Keras Custom model to fix the error

Added 2 lines of code `import keras`,
`@keras.saving.register_keras_serializable()` on top of the Keras Custom model definition because it shows typeerror - "Make sure custom classes are decorated with `@keras.saving.register_keras_serializable()`" after reloading the model when saving as `.keras`.

As mentioned in this Saving [Custom objects](https://www.tensorflow.org/guide/keras/serialization_and_saving#custom_objects) definition.

Please refer to this replicated working code [gist](https://colab.research.google.com/gist/RenuPatelGoogle/f124503c52144c5ed6e9c79cffcb593d/intro_to_modules.ipynb#scrollTo=hpW-4Dj3Q1Rb) for your reference. Thank you.

site/en/guide/intro_to_modules.ipynb

@@ -918,6 +918,8 @@
       },
       "outputs": [],
       "source": [
+        "import keras
+        "@keras.saving.register_keras_serializable()
         "class MySequentialModel(tf.keras.Model):\n",
         "  def __init__(self, name=None, **kwargs):\n",
         "    super().__init__(**kwargs)\n",