docs: minor fix in the sample config for ESM (#6340)

The line `return config;` was left by mistake I guess. The function is already returning the `config` object.

website/docs/guides/markdown-features/markdown-features-math-equations.mdx

@@ -187,7 +187,6 @@ async function createConfig() {
     ],
     // highlight-end
   };
-  return config;
 }
 
 module.exports = createConfig;