Remove `show` class from example code (#25570)

docs/4.0/components/dropdowns.md

@@ -46,7 +46,7 @@ Any single `.btn` can be turned into a dropdown toggle with some markup changes.
 And with `<a>` elements:
 
 {% example html %}
-<div class="dropdown show">
+<div class="dropdown">
   <a class="btn btn-secondary dropdown-toggle" href="#" role="button" id="dropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
     Dropdown link
   </a>