Add missing space in docs

[aristath]

Description

Adds a missing space. Noticed while reading the docs on https://developer.wordpress.org/block-editor/how-to-guides/themes/theme-json/#backward-compatibility-with-add_theme_support

How has this been tested?

No test needed, this is a simply spacing issue to improve docs readability,

Screenshots

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

LGTM