[css-snappoints] Update figcaption for example 1

Author: ChumpChief

css-snappoints/Overview.bs

@@ -98,10 +98,7 @@ Ignored Terms: scroll-snap-positions-*, containing block chain
 
             <figcaption>
                 The layout of the scroll container's contents in the example.
-                The snap-destination is horizontally and vertically centered within the scroll container's visual viewport
-                (represented by a red X),
-                and each element has its snap coordinate horizontally and vertically centered within the element
-                (represented by yellow plus signs).
+                The snap alignment container is represented by the red rectangle, and the snap area is represented by the yellow rectangle.  Since the scroll-snap-align is "center" in the X axis, a snap position is established at each scroll offset which aligns the X-center of the snap alignment container (represented by a red dotted line) with the X-center of a snap area (represented by a yellow dotted line).
             </figcaption>
         </figure>
     </div>