Update article.md

"an arbitrary HTML" -> "arbitrary HTML"

or could have said "an arbitrary piece of HTML" or something like that.

Author: paroche

2-ui/1-document/07-modifying-document/article.md

@@ -199,7 +199,7 @@ For instance:
 <p>Bye</p>
 ```
 
-That's how we can append an arbitrary HTML to the page.
+That's how we can append arbitrary HTML to the page.
 
 Here's the picture of insertion variants: