It looks like you have both Guidelines and FAQ on the menu - I think that’s what makes it overflow. If I add FAQ to my site it does the same.
A little additional CSS can make it wrap:
.nav-pills {
flex-wrap: wrap;
}
Reducing the gap on .nav-pills might help a little too.
(This was just a quick test with browser dev tools. You might have to get more specific with the selectors if you don’t want to affect .nav-pills in other places.)