qapi: fix example of DUMP_COMPLETED event

qapi: fix example of DUMP_COMPLETED event

Example output lacks mandatory member @timestamp.  Provide it.

Example output is not properly formatted. Fixing it by:
 - Adding '<-' to signalize it is receiving the data;
 - Adding extra spaces around members @result, @total and @completed

Signed-off-by: Victor Toso <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: John Snow <[email protected]>
Message-Id: <20220328140604[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>