[css-writing-modes] Fix typo (Appendix C -> A)

https://lists.w3.org/Archives/Public/www-style/2016Mar/0402.html

Author: kojiishi

css-writing-modes/Overview.bs

@@ -1151,7 +1151,7 @@ Introduction to Vertical Text Layout</h2>
     i.e. one that is either vertical-only or that is bi-orientational.
     A <dfn>horizontal script</dfn> is one that has a native horizontal orientation:
     i.e. one that is either horizontal-only or that is bi-orientational.
-    (See <a href="#script-orientations">Appendix C</a> for a categorization of
+    (See <a href="#script-orientations">Appendix A</a> for a categorization of
     scripts by native orientation.)
 
   <div class="figure">