[css-pseudo-4] Fix formatting and error in example.

display: block is needed for opacity to work; the default is display:
contents.

Author: dbaron

css-pseudo-4/Overview.bs

@@ -1326,7 +1326,9 @@ Expandable contents of details element: the ''::details-content'' pseudo-element
     animate the opacity of the additional information
     when the <{details}> element opens:
 
-    <pre class="lang-css">details::details-content {
+    <pre class="lang-css">
+details::details-content {
+  display: block;
   opacity: 0;
   transition: content-visibility 300ms allow-discrete step-end, opacity 300ms;
 }