Fix broken links in v4-dev

@mdo: there are 2 internal broken links in v4-dev

Getting links from: http://localhost:3000/docs/4.0/examples/navbars/
├───OK─── http://localhost:3000/docs/4.0/examples/navbars/navbar.css
├─BROKEN─ http://localhost:3000/docs/4.0/examples/navbar-top/ (HTTP_404)
├─BROKEN─ http://localhost:3000/docs/4.0/examples/navbar-top-fixed/ (HTTP_404)
Finished! 82 links found. 79 excluded. 2 broken.

Now, apart from fixing them, I see the text for navbar-static says:

This example is a quick exercise to illustrate how the top-aligned navbar works. As you scroll, this navbar remains in its original position and moves with the rest of the page.

That seems like a bad copy-paste.