It could be that the “About us” element has a negative top-margin to push it up and a white background, blocking out part of the border.
Another way would be a gradient or image in the top border.
If you want to see the background behind the text then use the method that Leus Maximus showed above as that allows for images or gradients to be behind the text as you don’t need to rub anything out.
We had a similar question a few weeks ago and my answer was much the same.
If the background isn’t an issue then SamA74’s method is fine but maybe use ems instead of pixels for the negative margin in case the user resizes the text.