Update specificity explanation for :root pseudo-class (mdn#42224)

* Update specificity explanation for :root pseudo-class

Clarified specificity details of the :root pseudo-class.

* Update files/en-us/web/css/reference/selectors/_colon_root/index.md

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Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

Author: zhangenming

files/en-us/web/css/reference/selectors/_colon_root/index.md

@@ -6,7 +6,7 @@ browser-compat: css.selectors.root
 sidebar: cssref
 ---
 
-The **`:root`** [CSS](/en-US/docs/Web/CSS) [pseudo-class](/en-US/docs/Web/CSS/Reference/Selectors/Pseudo-classes) matches the root element of a tree representing the document. In HTML, `:root` represents the {{HTMLElement("html")}} element and is identical to the selector `html`, except that its [specificity](/en-US/docs/Web/CSS/Guides/Cascade/Specificity) is higher.
+The **`:root`** [CSS](/en-US/docs/Web/CSS) [pseudo-class](/en-US/docs/Web/CSS/Reference/Selectors/Pseudo-classes) matches the root element of a tree representing the document. In HTML, `:root` represents the {{HTMLElement("html")}} element and is identical to the selector `html`, except that its [specificity](/en-US/docs/Web/CSS/Guides/Cascade/Specificity) (0-1-0) is higher than html's (0-0-1).
 
 ```css
 /* Selects the root element of the document: