Doesn't look like there is an option to display the summary.
The code to display the course name and link is in
function course_overview() in /blocks/course_overview/renderer.php
If you look for $coursefullname, you should see something like this
$coursefullname = format_string($course->fullname, true, $course->id);
$link = html_writer::link($courseurl, $coursefullname, $attributes);
$html .= $this->output->heading($link, 2, 'title');
So you'll need to add something like this
$html .= $course->summary;
The $course->summary normally contains a lot of html, if you want to remove that then use this instead
$html .= format_string($course->summary);