Fixed typo in article.md

Fixed small typo in 'Page: DOMContentLoaded, load, beforeunload, unload' article.md

Author: dashaezhova

2-ui/5-loading/01-onload-ondomcontentloaded/article.md

@@ -173,7 +173,7 @@ window.onbeforeunload = function() {
 };
 ```
 
-For historical reasons, returning a non-empty string also counts as canceling the event. Some time ago browsers used show it as a message, but as the [modern specification](https://html.spec.whatwg.org/#unloading-documents) says, they shouldn't.
+For historical reasons, returning a non-empty string also counts as canceling the event. Some time ago browsers used to show it as a message, but as the [modern specification](https://html.spec.whatwg.org/#unloading-documents) says, they shouldn't.
 
 Here's an example: