[css-ui][editorial] Explicitly define <x> and <y> as types

Author: cdoublev

css-ui-4/Overview.bs

@@ -827,8 +827,8 @@ Styling the Cursor: the 'cursor' property</h4>
 					The optional &lt;x&gt; and &lt;y&gt; coordinates
 					identify the exact position within the image which is the pointer position (i.e., the hotspot).
 
-				<dt><dfn><<x>></dfn>
-				<dt><dfn><<y>></dfn>
+				<dt><dfn type><<x>></dfn>
+				<dt><dfn type><<y>></dfn>
 				<dd>
 					Each is a <<number>>.
 					The x-coordinate and y-coordinate of the position