[css-text-decor-3] Fix broken url for image

Didn't realized that this image was used multiple times in the document
when updating it earlier.

Closes w3c#2699

Author: frivoal

css-text-decor-3/Overview.bs

@@ -841,7 +841,7 @@ Emphasis Mark Position: the 'text-emphasis-position' property</h3>
           <td rowspan="3">
               <img alt="Emphasis marks appear over each emphasized character in horizontal Japanese text."
                   title="Emphasis (shown in blue for clarity) applied above a fragment of Japanese text"
-                  src="images/text-emphasis-ja.gif" height="28" width="225">
+                  src="images/text-emphasis-ja.png" height="39" width="208">
 
           <td rowspan="4">
               <img alt="Emphasis marks appear on the right of each emphasized character in vertical Japanese text."