qapi: fix example of MEM_UNPLUG_ERROR event

Example output was missing ',' delimiter. Fix it.

Problem was noticed when trying to load the example into python's json
library.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Message-Id: <20220901085840.22520-8-victortoso@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>