package(prettier-config): update docs

[thelovekesh]

What?

• Fix doc for extending prettier-config in .prettierrc file.
• Add step for extending prettier-config in .prettierrc.js file.

Why?

Adding extends: ['@wordpress/prettier-config'] in prettierrc file doesn't really extends the config.

Also, see: https://stackoverflow.com/questions/63963613/configuring-prettier-format-tool-for-wordpress-in-prettierrc

How?

- extends: ['@wordpress/prettier-config']
+ "@wordpress/prettier-config"

Testing Instructions

Try extending the config with both ways inside the code editor and open the extension terminal by:

• Open the terminal
• Select the output tab
• Choose prettier from the dropdown on the right side.
• Now see the output in both cases.

[github-actions]

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[gziolo]

I can confirm this is a correct update. I used the same syntax in one of my test projects:

https://github.com/gziolo/wp-block-directory-template/blob/ea020225ef13881eb730550ae0f7c7c57f8944cb/package.json#L56