[css-syntax] Two dash <delim-token>s must be serialized as -/**/-

... now that `--` is an identifier:
http://hg.csswg.org/drafts/rev/ab8b75f1fa88

Author: SimonSapin

css-syntax/Overview.bs

@@ -3354,7 +3354,7 @@ Serialization</h2>
 				<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td> <td> <td>
 			<tr>
 				<th>-
-				<td>✗<td>✗<td>✗<td>✗<td> <td>✗<td>✗<td>✗<td> <td> <td>
+				<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td>✗<td> <td> <td>
 			<tr>
 				<th>number
 				<td>✗<td>✗<td>✗<td>✗<td> <td>✗<td>✗<td>✗<td> <td> <td>