Use `navbar-expand-sm` in our navbar bottom example for small devices (#24463)

docs/4.0/examples/navbar-bottom/index.html

@@ -21,7 +21,7 @@
         <a class="btn btn-lg btn-primary" href="../../components/navbar/" role="button">View navbar docs &raquo;</a>
       </div>
     </div>
-    <nav class="navbar fixed-bottom navbar-expand navbar-dark bg-dark">
+    <nav class="navbar fixed-bottom navbar-expand-sm navbar-dark bg-dark">
       <a class="navbar-brand" href="#">Bottom navbar</a>
       <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
         <span class="navbar-toggler-icon"></span>