Update bottom border on dark navbar example (twbs#38707)

* Docs: drop bottom border from dark navbar example

* Use `.border-body`

site/content/docs/5.3/components/navbar.md

@@ -331,7 +331,7 @@ Mix and match with other components and utilities as needed.
 Navbar themes are easier than ever thanks to Bootstrap's combination of Sass and CSS variables. The default is our "light navbar" for use with light background colors, but you can also apply `data-bs-theme="dark"` to the `.navbar` parent for dark background colors. Then, customize with `.bg-*` and additional utilities.
 
 <div class="bd-example">
-  <nav class="navbar navbar-expand-lg bg-dark border-bottom border-bottom-dark" data-bs-theme="dark">
+  <nav class="navbar navbar-expand-lg bg-dark border-bottom border-body" data-bs-theme="dark">
     <div class="container-fluid">
       <a class="navbar-brand" href="#">Navbar</a>
       <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarColor01" aria-controls="navbarColor01" aria-expanded="false" aria-label="Toggle navigation">
@@ -420,7 +420,7 @@ Navbar themes are easier than ever thanks to Bootstrap's combination of Sass and
 </div>
 
 ```html
-<nav class="navbar bg-dark border-bottom border-bottom-dark" data-bs-theme="dark">
+<nav class="navbar bg-dark border-bottom border-body" data-bs-theme="dark">
   <!-- Navbar content -->
 </nav>