Fix nested paragraphs on event list

The event list on /about/events/ fails validation due to nesting a block-level
element (in this case a <p>) inside a <p> paragraph. The <p> element is a
block-level element which prohibits any other nested block-level elements such
as <div> but also <p> itself. The culprit is the markdown filter which renders
using <p> elements.

Attached patch replace the <p> element wrapping the rendered event summary with
a <div> instead. Since browsers close the currently open <p> when encountering
a block-level element, this should not introduce any visible changes but since
I don’t have a staging environment it needs testing.

cheers ./daniel