Twenty Twenty: Remove extra space from `arrow-down-circled` icon name in...

Twenty Twenty: Remove extra space from `arrow-down-circled` icon name in `TwentyTwenty_SVG_Icons::$ui_icons`.

Props kitchin.
Fixes #49091.
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git-svn-id: https://core.svn.wordpress.org/trunk@46819 1a063a9b-81f0-0310-95a4-ce76da25c4cd