Fix typo in Slot Fills documentation

[greenworld]

Fix typo in code example for section "Restricting fills to the Side Editor".

What?

Fixes error in code example provided in documentation for Slot Fills.

Why?

Current example was not working for me, the correct way is the same as in the section "Restricting fills to the Post Editor" above.

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

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

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

[t-hamano]

Good catch 👍

[github-actions]

Congratulations on your first merged pull request, @greenworld! We'd like to credit you for your contribution in the post announcing the next WordPress release, but we can't find a WordPress.org profile associated with your GitHub account. When you have a moment, visit the following URL and click "link your GitHub account" under "GitHub Username" to link your accounts:

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Kudos!