Fix typo (mdn#40855)

Author: codersjj

files/en-us/web/css/_doublecolon_before/index.md

@@ -237,7 +237,7 @@ li[aria-current="step"]::before {
 
 ### `::before::marker` nested pseudo-elements
 
-The `::before::marker` [nested pseudo-element](/en-US/docs/Web/CSS/Pseudo-elements#nesting_pseudo-elements) selects the list {{CSSxRef("::marker")}} of an `::after` pseudo-element that is itself a list item, that is, it has its {{CSSxRef("display")}} property set to `list-item`.
+The `::before::marker` [nested pseudo-element](/en-US/docs/Web/CSS/Pseudo-elements#nesting_pseudo-elements) selects the list {{CSSxRef("::marker")}} of a `::before` pseudo-element that is itself a list item, that is, it has its {{CSSxRef("display")}} property set to `list-item`.
 
 In this demo, we generate extra [list items](/en-US/docs/Web/HTML/Reference/Elements/li) before and after a list navigation menu using `::before` and `::after` (setting them to `display: list-item` so they behave like list items). We then use `ul::before::marker` and `ul::after::marker` to give their list markers a different color.