[css-backgrounds-3] Fix typo in bg-position grammar. w3c#5104

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -581,7 +581,7 @@ layer.
     <tr>
       <th><a href="#values">Value</a>:
 
-      <td><<bg-position>>
+      <td><<bg-position>>#
     <tr>
       <th>Initial:
       <td>0% 0%