docs: fix type syntax in _gutenberg_add_block_template_plugin_attribute()

[justlevine]

What?

This PR fixes the PHPDoc type syntax used in _gutenberg_add_block_template_plugin_attribute().

Why?

It seems to have been committing using JSDoc syntax.

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

[github-actions]

Warning: Type of PR label mismatch

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Technical Prototype, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core, Gutenberg Plugin.
• Labels found: .

Read more about Type labels in Gutenberg. Don't worry if you don't have the required permissions to add labels; the PR reviewer should be able to help with the task.

[github-actions]

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

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