Update attributes example: alter source children

[rflw]

What?

Update attributes guide https://developer.wordpress.org/block-editor/how-to-guides/block-tutorial/introducing-attributes-and-editable-fields/.

Why?

There's no children value mentioned for source in official reference https://developer.wordpress.org/block-editor/reference-guides/block-api/block-attributes/#value-source.

Available values are only:

– (no value) – when no source is specified then data is stored in the block’s comment delimiter.
– attribute – data is stored in an HTML element attribute.
– text – data is stored in HTML text.
– html – data is stored in HTML. This is typically used by RichText.
– query – data is stored as an array of objects.
– meta – data is stored in post meta (deprecated).

How?

All examples should be replaced by valid source value or attributes documentation should be updated to add children value as official.

Testing Instructions

1. Open https://developer.wordpress.org/block-editor/how-to-guides/block-tutorial/introducing-attributes-and-editable-fields/
2. Go to first code snippet

attributes: {
    content: {
        type: 'array',
        source: 'children',
        selector: 'p',
    },
},

3. Compare source value with official documentation https://developer.wordpress.org/block-editor/reference-guides/block-api/block-attributes/#value-source
   There's no source: children mentioned.

Screenshots or screencast

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @rflw! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[Mamaduka]

Thanks for contributing, @rflw.

The changes look good. I just left a small suggestion.

[github-actions]

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