[css-writing-modes] Editorial: fix alignment in figure

Closes #4273

Author: frivoal

css-writing-modes-3/Overview.bs

@@ -1273,9 +1273,9 @@ Orienting Text: the 'text-orientation' property</h3>
         </td>
       </tr>
       <tr>
-        <td>''mixed''</td>
-        <td>''upright''</td>
-        <td>''sideways''</td>
+        <td style="text-align:center">''mixed''</td>
+        <td style="text-align:center">''upright''</td>
+        <td style="text-align:center">''sideways''</td>
       </tr>
     </table>
     <p class="caption">'text-orientation' values ('writing-mode' is ''vertical-rl'')</p>

css-writing-modes-4/Overview.bs

@@ -1348,9 +1348,9 @@ Orienting Text: the 'text-orientation' property</h3>
         </td>
       </tr>
       <tr>
-        <td>''mixed''</td>
-        <td>''upright''</td>
-        <td>''sideways''</td>
+        <td style="text-align:center">''mixed''</td>
+        <td style="text-align:center">''upright''</td>
+        <td style="text-align:center">''sideways''</td>
       </tr>
     </table>
     <p class="caption">'text-orientation' values ('writing-mode' is ''vertical-rl'')</p>