Fix navbarBurger tag from <a> to <div> avoiding page reload.

When following the example provided in the documentation, clicking on the
burger triggers a page reload. As suggested in an issue on the official
Bulma GitHub repository (jgthms/bulma#1515),
changing the <a> to a <div> fixes the problem.

src/Bulma/Components.elm

@@ -1073,7 +1073,7 @@ When its first argument is `True`, it transforms into a `navbarCross`.
 
 -}
 navbarBurger : IsActive -> List (Attribute msg) -> List (Html msg) -> NavbarBurger msg
-navbarBurger isActive = node "a" [ B.navbarBurger, if isActive then B.isActive else B.none ]
+navbarBurger isActive = node "div" [ B.navbarBurger, if isActive then B.isActive else B.none ]
 
 {-| A simple "X" character; the active version of `navbarBurger`.
 -}