Update nav-tab.md

[jitangupta]

Updated the anchor tag with button in sample examples

[XhmikosR]

@mdo @ffoodd: we use both a and buttons in the markup examples, are you sure it makes sense to change the JS API examples?

[ffoodd]

I'd recommend to always use buttons for tabs, so yes. Might we replace as with buttons everywhere, too?

[XhmikosR]

I'd just change this patch to indicate that both can be used for now using a JS comment, since both markups are allowed.

[mdo]

Going to merge as-is and we can iterate on this further with additional PRs.