[css-contain] Typo: dot in the middle of sentence (#2619)

Author: Loirooriol

css-contain-1/Overview.bs

@@ -342,7 +342,7 @@ Paint Containment</h3>
 	Otherwise, giving an element <dfn export>paint containment</dfn> has the following effects:
 
 	1. The contents of the element
-		including both the paint of the descendants and their geometry.
+		including both the paint of the descendants and their geometry
 		must be clipped to the <a>padding edge</a> of the element's <a>principal box</a>,
 		taking <a>corner clipping</a> into account.
 		This does not include the creation of any mechanism