Added :preferred_syntax to the configuration reference

doc-src/content/tutorials/configuration-reference.markdown

@@ -218,6 +218,13 @@ command line will override the corresponding settings in your configuration file
       in production mode, true in development mode.
     </td>
   </tr>
+  <tr>
+    <td style="vertical-align:top;"><code>preferred_syntax</code> </td>
+    <td style="vertical-align:top;">Symbol </td>
+    <td style="vertical-align:top;">Can be <code>:scss</code> or <code>:sass</code>.
+      Defaults to <code>:scss</code>.
+    </td>
+  </tr>
 </table>
 
 ## Configuration Functions