Fix navbar separator on mobile sizes

Before, the separator stayed as vertical line in mobile orientation.
Now, it's replaced with dropdown separator (i.e., horizontal line).

Signed-off-by: Marek Pikuła <marek@serenitycode.dev>

layouts/partials/sections/header.html

@@ -140,7 +140,8 @@
                     {{ end }}
 
                     {{ if and .Site.Menus.main (.Site.Params.navbar.enableSeparator | default false) }}
-                        <li class="nav-item navbar-text"><div class="nav-link nav-link-sep"></div></li>
+                        <li class="nav-item navbar-text d-none d-lg-block d-xl-block"><div class="nav-link nav-link-sep"></div></li>
+                        <div class="dropdown-divider"></div>
                     {{ end }}
 
                     <!-- custom menus from the user -->