docs: fix $metadata param type in WP_Theme_JSON_Gutenberg::get_feature_declarations_for_node()

[justlevine]

What?

This PR fixes the $metadata param type for WP_Theme_JSON_Gutenberg::get_feature_declarations_for_node() so it correctly expects an array type instead of a object.

Why?

While this issue was surfaced by PHPStan (via #66693) it can be remediated independently as part of #66598

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

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Co-authored-by: justlevine <[email protected]>

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