[css-writing-modes-3] Fix table markup error.

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -1708,7 +1708,7 @@ Abstract-to-Physical Mappings</h3>
   <table class="complex data">
     <caption>Abstract-Physical Mapping</caption>
     <colgroup class="header"></colgroup>
-    <colgroup span=8></colgroup>
+    <colgroup span=6></colgroup>
     <thead>
       <tr>
         <th scope="row">'writing-mode'</th>