qapi: fix example of BLOCK_IO_ERROR event

Example output lacks mandatory member @reason.  Provide it.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Message-Id: <20220328140604.41484-4-victortoso@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>