docs: clarify Tutorial Part 7

[adamu]

Added a clarification to avoid a common error when following the tutorial, which is to forget to add getNode to the function parameters.

This happeed to me, and looks like it's happened to a few others too.

Hopefully this will help avoid other people making the same mistake.

[gillkyle]

Thanks @adamu! Seeing the StackOverflow thread helps highlight how this change will be helpful.

I think this additional clarity is great 🎉

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