Update nav-main.html

Navbars have also gone under major structural change. Here's a summary of what we're doing:

Replace .navbar-search with .navbar-form
Replace .navbar-inner with .container
Replace .navbar .nav with .navbar-nav
.brand is now .navbar-brand
.navbar.pull-left is now .navbar-left
.navbar.pull-right is now .navbar-right
.nav-collapse is now .navbar-collapse
.navbar-brand, .navbar-toggle are wrapped by .navbar-header
.navbar:not(.navbar-inverse) is now .navbar.navbar-default