fix typo

Author: avivkeller

packages/mdx/lib/__tests__/shiki.test.mjs

@@ -3,7 +3,7 @@ import { mock, it, describe } from 'node:test';
 
 import { SKIP } from 'unist-util-visit';
 
-mock.module('classNames', {
+mock.module('classnames', {
   defaultExport: (...args) => args.filter(Boolean).join(' '),
 });