[css-text-decor-3] Tweak example to make it more typical

Rationale by Kobayashi-sensei:

> https://drafts.csswg.org/css-text-decor-3/#emphasis-marksの冒頭にある
> 圏点のついた図の例ですが，日本語組版の横組で，あのような形には一般にし
> ません．JLReqの図3. 88に示したように一般に縦組で使う記号と横組で使う
> 記号は異なります．図3. 88の下側のような記号に直した方が望ましいで
> しょう．

Or in other words, the type of emphasis mark shown in the example is
more typical of vertical text. Should replace the sesame with a circle,
which is typical of horizontal text. See JLReq figure 3.88 for an
illustration.

Author: frivoal

css-text-decor-3/Overview.bs

@@ -627,7 +627,7 @@ Emphasis Marks</h2>
   <div class="figure">
     <p>
       <img alt="Example of emphasis in Japanese appearing over the text"
-           class="example" src="images/text-emphasis-ja.gif" height="28" width="225"></p>
+           src="images/text-emphasis-ja.png" height="39" width="208"></p>
     <p class="caption">Accent emphasis (shown in blue for clarity) applied to Japanese text</p>
   </div>