Under Settings > URLs and SEO > SEO – URLs you can find the setting Default page. With the option “Use domain root“.
This will use your domain root when going to your websites basic domain.
However if you locate your other pages below the home-page node. It will include this by default in the node alias path.
You can manually change the path for this one page. But I guess its easier to not place your about-us page beneath the home node, but as a sibling.