[css-flexbox-1] Increment appendix numbering

Author: fantasai

css-flexbox-1/Overview.bs

@@ -3611,7 +3611,7 @@ Appendix A: Axis Mappings</h2>
 	</table>
 
 <h2 class="no-num" id="webkit-aliases">
-Appendix A: <code>-webkit-</code> Legacy Properties</h2>
+Appendix B: <code>-webkit-</code> Legacy Properties</h2>
 
 	<em>This appendix is normative.</em>