Replace outdated reference to <geometry-box> in mask-clip (#31433)

Since w3c/fxtf-drafts#439 and the related pull request were closed, mask-clip uses <coord-box> instead of <geometry-box>. This was mostly fixed in this #28157 previous pull request but I forgot to update this comment as well.

files/en-us/web/css/mask-clip/index.md

@@ -12,7 +12,7 @@ The **`mask-clip`** [CSS](/en-US/docs/Web/CSS) property determines the area whic
 ## Syntax
 
 ```css
-/* <geometry-box> values */
+/* <coord-box> values */
 mask-clip: content-box;
 mask-clip: padding-box;
 mask-clip: border-box;