The header you're seeing there is because GitHub Pages uses the Primer theme by default to publish your Pages site:
Primer will include your repository's name in an <h1> tag if your page's title is different from your site's title (or in this case, your page has no title):
If you'd like the remove that header, you'll want to customize your Jekyll theme's HTML. In order to do that you'll need to copy this file into your repository inside a folder called _layouts:
After you've done that, you can remove lines 12-14 to remove the header, and Jekyll will then use that file as a template instead of the default template that comes with the theme.