[TYPO][Chap 2.2]: The fourth instead of the fifth

[toutane]

At the end of the "Key Attributes" section, just before figure 2.2, it is written that we display the fourth element, but it's the fifth:

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Let’s display the fourth digit in this rank-3 tensor, using the library Matplotlib (part of the standard scientific Python suite); see figure 2.2.
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digit = train_images[4]
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