Fix Keras Model submodules attribute error in intro_to_modules guide

[sanjana7668-eng]

Summary of Changes

Replaced .submodules with .layers on the my_sequential_model instance in intro_to_modules.ipynb, as tf.keras.Model objects do not have a submodules attribute.

Closes tensorflow/tensorflow#97403

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[github-actions]

Preview

Preview and run these notebook edits with Google Colab:

• site/en/guide/intro_to_modules.ipynb

Rendered notebook diffs available on ReviewNB.com.

Format and style

Use the TensorFlow docs notebook tools to format for consistent source diffs and lint for style:

$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs

$ python3 -m tensorflow_docs.tools.nbfmt notebook.ipynb

$ python3 -m tensorflow_docs.tools.nblint --arg=repo:tensorflow/docs notebook.ipynb

If commits are added to the pull request, synchronize your local branch: git pull origin sanjana7668-eng-patch-1