Hey Julie,
This is primarily because the text is too long and you are probably viewing it with a much smaller screen. Since there is padding on the outer wrap of protostar, there isn't enough screen estate here.
What I would suggest is to remove the padding on the protostar template since you are already having an extremely small viewport, doesn't make sense to add too much whitespace on the template. You could add the block of css codes into /templates/protostar/css/user.css
[gist]
@media (max-width: 480px) {
body {
padding: 0 !important;
}
}
[/gist]