Docs: Copy and formatting edits for the "block.json" guide

[ndiego]

This PR focused on formatting and copy edits for the block.json guide. The goal is to fix a few formatting issues and improve readability.

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Props ndiego, greenshady.

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Co-authored-by: ndiego <ndiego@git.wordpress.org>
Co-authored-by: justintadlock <greenshady@git.wordpress.org>

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