REST API: Fix array syntax after [43851].

I thought I was writing JavaScript. Alas.

Merges [43852] from the 5.0 branch to trunk.

Props danielbachhuber.
See #45196.

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git-svn-id: https://core.svn.wordpress.org/trunk@44048 1a063a9b-81f0-0310-95a4-ce76da25c4cd