simplified the answer to the updating data exercise

[AndyBette]

there's no need for --expression-attribute-names. I also added return-values to demonstrate that the command behaves as expected.

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Description of changes:

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[tebanieo]

Instead of removing it I think it will be clearer for users to understand the difference between both approaches and when they need to use # for expression attributes. Please add both explanations.

Don't forget to rebase your branch.