[css-backgrounds-3] Fix grammar error in background shorthand value definition introduced during switch to # multipliers

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1115,7 +1115,7 @@ layer.
       <td><dfn id="background">background</dfn>
     <tr>
       <th><a href="#values">Value</a>:
-      <td><<bg-layer>># , <<final-bg-layer>>
+      <td>[<<bg-layer>># ,]? <<final-bg-layer>>
     <tr>
       <th>Initial:
       <td>see individual properties