Fix custom menu

Reading the documentation, I found the method description and examples quite a
bit more simple than what I found in functions.php.

http://codex.wordpress.org/Function_Reference/register_nav_menu

The example they give is:

```php
add_action( 'after_setup_theme', 'register_my_menu' );
function register_my_menu() {
  register_nav_menu( 'primary', 'Primary Menu' );
}
```

So the `register_nav_menu` function is really straightforward. But you had this:

```php
register_nav_menu( 'top-nav' => __( 'Top Navigation Bar', 'OmahaFilmFestival' ) );
```

I just simplified it to be nearer to the example, and now it works as expected.