docs: fix syntax in block filters example

[chrisbellboy]

What?

Fixes the syntax in block filters example.

Why?

Incorrect syntax used in block filters example.

How?

Replaces incorrect curly braces with parentheses

[github-actions]

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[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @chrisbellboy! In case you missed it, we'd love to have you join us in our Slack community.

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[fabiankaegy]

Good catch 👍