Clarify confusing point

Author: smith558

1-js/11-async/01-callbacks/article.md

@@ -102,7 +102,7 @@ function loadScript(src, callback) {
 *!*
 loadScript('https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js', script => {
   alert(`Cool, the script ${script.src} is loaded`);
-  alert( _ ); // function declared in the loaded script
+  alert( _ ); // _ is a function declared in the loaded script
 });
 */!*
 ```