Navbar centered

basic.html

<nav class="navbar fixed-top navbar-expand-lg navbar-dark bg-primary">
  <a class="navbar-brand" href="#">Brand </a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#main_nav">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="main_nav">
    <ul class="navbar-nav mx-auto">
	<li class="nav-item active"> <a class="nav-link" href="#">Home </a> </li>
	<li class="nav-item"><a class="nav-link" href="#"> About </a></li>
	<li class="nav-item"><a class="nav-link" href="#"> Services </a></li>
    </ul>
    <a class="btn btn-warning" href="#">Button</a>
  </div> <!-- navbar-collapse.// -->
</nav>