Twenty Twenty: Fix typos in a variable name and inline comment in...

Twenty Twenty: Fix typos in a variable name and inline comment in `assets/js/color-calculations.js`.

Props tanvirul.
Fixes #48704.
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git-svn-id: https://core.svn.wordpress.org/trunk@46547 1a063a9b-81f0-0310-95a4-ce76da25c4cd