update instances of old classes in the docs

docs/_includes/nav-docs.html

@@ -1,4 +1,4 @@
-<form class="bd-search hidden-sm-down">
+<form class="bd-search d-none d-sm-block">
   <input type="search" class="form-control" id="search-input" placeholder="Search..." aria-label="Search for..." autocomplete="off">
   <div class="dropdown-menu bd-search-results" id="search-results"></div>
 </form>

docs/_includes/nav-home.html

@@ -25,7 +25,7 @@
     </nav>
     {% endcomment %}
 
-    <div class="d-flex justify-content-between hidden-lg-up">
+    <div class="d-flex justify-content-between d-lg-none">
       <a class="navbar-brand" href="{{ site.baseurl }}/">
         Bootstrap
       </a>